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Find Out Why Check Engine Light Is On and Avoid Costly Repairs


Glenn Stok applies his engineering background to solve technological problems and save money on car repairs. He shares his insight below.

Diagnostic Code Scanner

Diagnostic Code Scanner

If your "check engine light" is on and your mechanic says you need some expensive repair, there is an easy way for you to check if he is telling the truth. You can read your car's diagnostic trouble codes yourself with a low-cost Diagnostic Code Scanner that plugs in under the dash to access the car’s computer.

This article will show you how to understand what your check engine light means, whether there is a real problem or just a faulty sensor, how to reset the light, and potentially avoid expensive car repair bills.

There are many component failures that turn on the check engine light. One of the most expensive to repair is the catalytic converter. However, many times the light is triggered by nothing more than a faulty sensor.

Here's what you'll get from this article:

  • I'm going to explain how to read your car’s diagnostic codes.
  • I will also explain how to tell whether there is a real issue with your engine, or simply a faulty sensor that triggered the light.
  • I'll show you how to clear the light so that you can pass an inspection.
  • Some cars also have a "Check Tire Pressure" light. I'll discuss that too.
  • Finally, I conclude with a section listing common questions readers have been asking, along with my answers.

Understanding Your Car's Diagnostic Codes

Your car's computer, known as the Powertrain Control Module (PCM), keeps a record of the diagnostic codes that represent which sensor is reporting a problem.

The check engine light comes on when any sensor reports a faulty device. However, it may not be the device that is bad. It may just be an inexpensive faulty sensor.

When sensors fail, it is usually intermittent. A useful experiment is to clear the codes and watch to see if the check engine light comes back on. You can read your car’s diagnostic codes with a simple code scanner. It plugs into a special plug that you can find under the dash.

You can also use these scanners to reset the PCM by clearing any saved trouble codes.

Since I have a background in computer science, I am aware that the only way the PCM will know if a unit is failing is by a sensor reporting back to it.

When a code is stored, the check engine light is lit to let you know. A specific diagnostic code indicates which sensor reported the problem. These codes are known as the OBD-I and OBD-II codes, but more on that in a moment.

Sensors Can Fail and Report False Codes

Your car has sensors for many components. Most are related to emissions control (see below for more about the tire pressure warning light on newer cars).

Sometimes a sensor will fail or get stuck and report a false code. An honest mechanic will tell you that. Replacing a sensor is not that expensive.

You can do your own cheap engine diagnostics by using one of these low-cost code scanners. That will help you discover why your check engine light is on. But knowing if it's a sensor problem takes some extra work, as I'll explain.

Choosing a Diagnostic Code Scanner

Many times I have noticed the mechanic plugging in a code scanner under the dash to read the engine trouble codes from the car's computer. I thought how nice it would be to know why the check engine light is on before I go into the repair shop. So I decided to spend a little money on my very own Diagnostic Code Scanner.

I did a few searches for these units and found reasonable prices and a wide choice of models on Amazon. Prices range from around $50 to the upper $200's depending on the model and features. After a little review, I realized that I didn't need to buy the most expensive one.

If you have a 1996 or newer car, then the cheaper models of Diagnostic Code Scanners will do just fine. All vehicles since 1996 use the OBD-II codes. The more expensive Code Scanners can read the older OBD-I codes as well. That is not necessary to have unless you have an older car built before 1996.

Why It's Worth Getting an OBD Code Scanner

Replacing the catalytic converter can be expensive for parts and labor combined. If you have your annual State Inspection coming up, then you will be forced to pay for the repair to pass the inspection.

The catalytic converter is part of the automobile exhaust system, placed between the engine and the muffler. It reduces the toxicity of emissions from internal combustion engines.

It was first introduced in the U.S. in the mid-1970s to comply with EPA regulations for controlling auto exhaust. Its functions are monitored today by the car's computer system with sensors that are on all important auto parts.

The sensor that detects a problem with the catalytic converter is known to trip erroneously sometimes. It happened to me and to several friends. I never needed a replacement because it was just a tripped sensor causing the check engine light to come on.

The catalytic converter typically has a warranty exceeding the rest of the automobile's warranty length. However, if you are beyond that warranty period (by time or by mileage), then you could be facing a large repair bill.

If you're in a state that implements vehicle emission inspection programs, you might also get stuck with needing it repaired quickly because your car failed.

You may be able to avoid replacing the catalytic converter. In many cases, it's just a bad sensor, and you wouldn't know that unless you can clear the car's diagnostic codes and watch to see if the check engine light comes back on.

The Diagnostic Scanner I Use

How to Read Your Code Scanner

This scanner that I use doesn't require me to look up the meaning of the codes. It shows the description of the codes on the screen. It also lets you reset the check engine light by clearing the codes from the computer.

Most diagnostic scanners offer similar features. Actron has versions that also read OBD-I codes in older vehicles with an optional cable. By reading the codes myself, I was able to see that my problem was indeed the catalytic converter.

Each code scanner is different, but they all show you the standard problem codes and have a function that allows clearing and resetting the computer. I recommend that you read the user's manual. Most good scanners will guide you through the process on the little screen if you carefully follow along with the on-screen prompts.

Video on Trouble Code Retrieval

How to Tell If You Really Have an Engine Problem

There is no way to know right away if the light is on due to a faulty sensor. You need to erase the code and then see if it comes back on eventually.

My scanner gave me all the features I needed to read the codes and erase them. I first chose the option to scan for any trouble codes in the computer. My scanner also shows the meaning of the codes in plain English. It's good to know this before erasing the codes. Then I select the option to erase the codes from the computer.

But you're not done yet! To know for sure whether the issue is a faulty sensor or a catalytic converter that needs repair, you need to follow a few more steps.

Diagnosing the Problem

After you reset the codes, either of two things will happen.

  1. If the "Not Ready" status becomes "Ready," and the check engine light did not come back on, then the problem may have been an intermittent faulty sensor.
  2. If the check engine light does come back on, then the scanner will show you that bad code again. That would indicate you do indeed have a component failure as specified by the code. In that case, at least you'll know you will be spending money to fix a real problem.

In case you're wondering whether you blew the light by turning it off, that is definitely not the case. First of all, the Diagnostic Code Scanner checks the condition of the check engine light as well. Secondly, when you turn on the ignition without starting the engine, all the panel lights light up temporarily as a test, so you can see they all are working.

Passing the Annual State Inspection

You will not pass inspection if the light is not off. That is accomplished by clearing the codes from the computer. But to pass inspection, the computer also has to show a "ready status." If you were to bring your car in for an inspection, the mechanic would tell you that he or she can't pass the vehicle at that time.

When you clear the codes with the “Erase Codes” function, the status of the System Monitors is set to “Not Ready.” You have to drive 50 to 100 miles in several individual trips until the system reads the status of all the components again.

“Several trips” means that you shut off the engine and start another trip. So it's not just 100 miles in one trip. It's more like making three trips of about 30 miles each.

Then connect the scanner again and read the System Status to see if the "Not Ready" condition is gone for all codes that you had cleared.

Three System Status Conditions You Need to Understand

System Status Explanation


If the check engine light did not come back on, the system is working properly and your car will pass emissions tests.


Your car has not been driven enough after resetting the codes. You need to drive until the system indicates a READY condition.


This means that your car does not support that status monitor and you don't need to be concerned about it.

Do you have a question?

See "Questions From Readers" in the last section below, along with the answers.

High-End Code Scanner With More Features

Some people prefer to spend more money to have many more features that replicate what can be done by service mechanics. I don't find any need for this just to monitor our own issues when the check engine light comes one.

With a simple scanner like the ones I mentioned above, you can get a good idea of the problem, and then bring your car in for a more precise checkup by a mechanic you trust.

Nevertheless, if you want a scanner that does a lot more, the INNOVA 3160 may be what you'll want to have. However, I'm satisfied with the one I use (mentioned above), which is under $50.

Understanding the Tire Pressure Warning Light

In 2008 the National Highway Traffic Safety Administration introduced a new requirement that all new cars and light trucks must have a Tire Pressure Monitoring System (TPMS).

These newer cars and trucks have a TPMS warning light in the dash, which will come on if the pressure on any of the tires is below 25% of the recommended pressure. A flashing or blinking TPMS light means that one of the TPMS sensors is malfunctioning.

The TPMS codes are not included in the ODB I or II codes and will not be recognized by an OBD scanner. A company named Accutire makes a tire pressure monitor specifically for the purpose of reading the TPMS codes.

Accutire MS-4378GB Tire Pressure Monitor

Accutire MS-4378GB Tire Pressure Monitor

This scanner displays diagnostics results of Sensor ID, Tire Pressure, Temperature, Battery Condition, and OE part number. Unlike the method of reading the OBD codes, no physical connection is required to read the status of the TPMS. It reads the data via wireless signals.

There are two methods for the TPMS system to work:

  1. One way is indirect by using the ABS/wheel-speed sensors (wheels spin at different speeds based on their inflation).
  2. The other method is a direct valve sensor.

If your “Tire Pressure Light” is on, check the pressure in each tire. The recommended inflation pressure for most passenger car tires is 32 to 34 PSI when cold. But check your car's owner's manual for its proper pressure.

Drive a while after properly inflating your tires. That will give the system a chance to register that the pressure is okay.

If the TPMS light does not go off after driving a while with the correct inflation, or if it's flashing, it could mean a fault with the tire pressure sensor, or with some other part of the monitoring system. You will need to find out what error code you have.

Some after-market wheels are known to have an issue. The sensor will not fit properly if the valve hole is in the middle of the wheel’s barrel. If you are buying new wheels, check with your dealer to see if they have wheels that are TPMS-compatible.

Questions From Readers About the Check Engine Light

These are common questions people have been asking, along with my answers.

Does the check engine light eventually turn off without fixing the problem?

If the problem is not due to an actual component, but rather just a faulty sensor that is intermittent, then you may notice the light goes off after some time. But it may come back on again once in a while if the sensor continues to send faulty signals.

It's best to have a mechanic look at it. But tell them that it seems to be a faulty sensor, so they know you already have some knowledge of the situation.

I know two friends who paid dearly for a new expensive part, only to have the light come back on weeks later. The mechanic refused to refund the cost, stating that he had no way of knowing it was only a bad sensor. Don't fall for that.

Is the catalytic converter connected to the car's computer?

Yes, that's one of the parts that is monitored by the Powertrain Control Module (PCM), the car's computer. If the Catalyst System Efficiency is below the threshold then it registers error code P0420.

Can the Actron scanner read individual sensors?

Yes, it will read all sensors and display the results. Then it lets you optionally clear the codes from the computer to start fresh.

How many miles need to be driven to reset check engine light for inspection?

After you clear the computer, you need to drive up to 100 miles to give it a chance to monitor all the sensors and register the results. Sometimes it may be completed in about 50 miles. You'll know when you connect the scanner and check the status. It tells you if ready or not ready. Wait until you see everything is in ready status before going for an inspection.

If you clear check engine codes with a scanner, will it allow you to pass inspection?

If the problem is a faulty part, the chances are good that the computer will read a new error and register a code again by the time you get into the shop for the inspection. The purpose of this article was not to cheat the system but to avoid costly repairs when only a sensor is at fault.

If it's only an intermittent problem with a sensor, it may take a while for the sensor to get stuck again, and you will pass inspection. Unless the sensor is so bad that it registers a code again before you get your car inspected. In any case, remember to wait until the computer registers "ready," as I mentioned in the last answer.

If I reset my light, will it show up in state inspection?

As long as you wait for the computer to show a "ready status," then there will be no clue that you reset the computer. However, if you do indeed have a problem, it will eventually register the code again and the light will come back on. See my prior answer about how many miles you need to drive before the ready condition is established.

I got a new catalytic converter, and my check engine light is still coming back on. Why?

When the check engine light comes on, and the scanner code shows that the catalytic converter is the cause, it could merely be the sensor that failed. That's why I recommend using your own scanner to clear the computer and see if the light comes back on. If the sensor is faulty, it may be intermittent.

When a car mechanic reads the code, they'll most likely sell you an expensive catalytic converter when all you needed was a new sensor. There is no way for them to know without clearing the computer and waiting to see what happens.

Is it possible that it could be a loose gas cap?

A loose gas cap causes a fuel vapor leak sensor to register code number P0455, and the check engine light will come on. But this code may be indicating a more serious problem. You could have damage anywhere in the EVAP system, which captures and returns the fuel vapor.

So, by all means, make sure your gas cap is tight. And if it's seal looks damaged, get a new gas cap. But if the light continues to come on and your scanner shows P0455, then get it checked out.

QUIZ: Test Your Knowledge of the Check Engine Light

For each question, choose the best answer. The answer key is below.

  1. When does the check engine light come on?
    • When the car is driven too fast.
    • When the car needs an oil change.
    • When a sensor indicates an emissions problem.
  2. The catalytic converter is part of...
    • The exhaust system.
    • The electrical system.
  3. What do OBD Diagnostic Codes indicate?
    • How well the driver is operating the vehicle.
    • They indicate which sensor is detecting a problem.
  4. When would a sensor report a faulty code in error?
    • When the computer gets too hot.
    • When a sensor gets stuck.
    • When the car was not maintained.
  5. The diagnostic trouble codes in the car's computer will remain forever.
    • True, they maintain a history.
    • False, once cleared and the part is fixed, there is no reference to the problem.
  6. A Diagnostic Code Scanner is used to...
    • Repair the problems reported with the OBD codes.
    • Read the codes from the car's computer.
    • Read the codes from the defective parts.
  7. Can a loose gas cap cause the check engine light to come on?
    • Yes
    • No

Answer Key

  1. When a sensor indicates an emissions problem.
  2. The exhaust system.
  3. They indicate which sensor is detecting a problem.
  4. When a sensor gets stuck.
  5. False, once cleared and the part is fixed, there is no reference to the problem.
  6. Read the codes from the car's computer.
  7. Yes

Interpreting Your Score

If you got between 0 and 2 correct answers: I was hoping you'd remember some of this stuff from the article.

If you got between 3 and 4 correct answers: I was hoping you'd remember some of this stuff from the article.

If you got 5 correct answers: You can do better.

If you got 6 correct answers: Very good!

If you got 7 correct answers: You're a pro!

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Questions & Answers

Question: The code "P0011" has appeared in my Nissan Micra 2012. It has 73000 km on it so far. Can you help?

Answer: That code refers to the Camshaft Position. It's the VVT (variable valve timing) or VCT (variable camshaft timing) components that are giving the error and need to be checked by a mechanic.

Question: Will the check engine light go off automatically if you change the faulty sensor?

Answer: The OBD diagnostic system will reset automatically after a while if the status clears up and is no longer detected. That may take up to over 100 miles of driving.

If you have it replaced by a mechanic, they will usually reset the status for you. You can always do this yourself with a low-cost diagnostic tool, such as the one I mentioned in my article.

If it is only the sensor that is faulty, the check engine light should not come back on. If it was not shut off by the mechanic, it should eventually reset once the OBD system no longer detects a problem.

Question: What do the OBD codes P0300 and P0339 mean?

Answer: You can do any search yourself by putting the code into Google along with the keyword “OBD code.” The two you asked about are as follows:

• The P0300 OBD code indicates “Random Misfire Detected” - This means not all cylinders are firing properly.

• The P0339 OBD code indicates “Crankshaft Position Sensor B Circuit Intermittent” - It means that the powertrain control module (PCM) is detecting an erratic voltage from sensor B, which is the secondary crankshaft position (CKP) sensor.

In both cases, you need to have your mechanic check your vehicle to determine if repairs are required.

Question: I'm getting code P0340 on my 2002 Oldsmobile Alero 2.2L, which indicates a camshaft position, sensor circuit malfunction. This engine doesn’t have a camshaft position sensor. I changed the crankshaft position sensor. I started the car before clearing the code. Will the sensor still show as bad in the PCM? What else should I do to fix this code so I can pass emissions?

Answer: In addition to indicating a problem with the Camshaft Position Sensor, the P0340 code could also be indicating a problem with the wiring from the sensor to the PCM. It could also be a bad PCM as well.

The sensor reads the data of the camshaft gears, so the P0340 code could also mean a timing problem. Therefore, you should check the timing belt. It can be worn or loose.

Question: Where on a 1990 Chevy pickup truck do you hook up the OBD-1 scanner?

Answer: OBD-1 diagnostics was used on GM vehicles from 1982 to 1995. The connector is under the dash on the driver's side. It may have a black plastic cover that you need to open for access to the plug.

Question: I have error code P0171, and my Pontiac is running rough and wasting gas. What’s the meaning of a Pontiac error code P0171?

Answer: Too much gasoline in the air-fuel ratio can cause the vehicle to run rough and waste gas. That condition will make the system look like it’s running too lean, and that will show a P0171 OBD error code. You need to have a mechanic diagnose the cause of the problem.

Question: What happens if you clear the codes by taking the battery off in your car?

Answer: That's the same as clearing the codes with the diagnostic scanner. The "Ready status" needs to be reestablished before your vehicle will pass the emissions test. The "ready status" occurs automatically after driving a reasonable distance as explained in the related article. The OBD error code will reappear and the check engine light will come back on if the original error is detected again.

© 2009 Glenn Stok

Reader Comments (Most Recent First)

Glenn Stok (author) from Long Island, NY on August 07, 2020:

Marion - Cheap gas could have water in it that can cause an error code with the catalytic converter. But you have no codes.

The only thing that can cause the check engine light to come on, and no codes are read, is a software error with the diagnostic scanner that your mechanic was using.

Since you ordered your own scanner, you'll soon know what codes are present.

Marion on August 07, 2020:

Hi, I recently bought an 2005 Mustang GT and the check engine light came on. My mechanic hooked up a scanner and no codes were given what so ever so he cleared the light for me.

Two weeks later the light is now back on again again, and no codes given.

Ive just ordered my own scanner and am waiting for it to arrive, but I’m curious if you have any thoughts on what’s going on.

My mechanic told me to drive it a good bit, just like you advised, and I didn’t exactly do that. I’ve barely driven it at all actually.

I did actually take it back to the original owner immediately after the first incident and he said he has always used cheap gas, and it drove fine. I don’t think there was any foul play on that end.

Any input from you would be super appreciated! Thanks!

Glenn Stok (author) from Long Island, NY on July 13, 2020:

Karen - There is no way I could tell you if the shop you use is honest and gives correct recommendations. You need to ask people locally who have used their service.

Karen on July 13, 2020:

I replaced my catalytic converter about 4 mos ago. I now put top tier fuel (shell 87 octane). Repair shop recommended a fuel system cleaning. I’m a bit on a budget. Do you think the shop recommendations is correct? I have a 2015 Kia Soul with 120,000+ miles.

Thank you for your advise

Glenn Stok (author) from Long Island, NY on July 05, 2020:

Kelly Gallagher - When the Diagnostic Codes are cleared, all are gone, but the PCM will indicate a "not ready" status until you've driven 50 to 100 miles without any return of codes.

Your diagnostic scanner will show you the ready status as I explained in this article, so you'll know when you can go for a state inspection.

I discussed all this in detail above. It will be helpful for you to read the entire article so you don't overlook anything crucial to know.

Kelly Gallagher on July 05, 2020:

Thank you for your response. I will indeed have the tb and injectors cleaned. Since that appointment is a few days away, I’ll likely drive the requisite 100 miles by then and should the light return it will be in the hands of the dealership shortly thereafter and they can then rescan for new codes and get a more accurate picture since my independent mechanic wiped them. On a related note, when a CEL is reset does it remove

ALL codes in the computer or just those the technician has cherry picked? The reason I ask is the dealership did supply me with a list of historic codes but they all predate this event.

Glenn Stok (author) from Long Island, NY on July 05, 2020:

Kelly Gallagher - You just answered your own question. The story about getting 18 error codes is a read flag that your mechanic can’t be trusted, especially since he or she didn’t want to give you the list of codes.

Trust that second opinion. That sounds like an honest one. At 80,000 miles it is fine to consider the small investment in maintenance that they suggested. But that $5,000 quote from your first mechanic is ridiculous.

Since the light never came back on, why worry. It’s possible that you merely had a bad tank of gas.

In any case, spend a little money to buy a diagnostic scanner for yourself, such as the one I use that I mentioned in this article. Then you’ll be able to see for yourself what error codes are responsible for the check engine light coming on when it does.

Kelly Gallagher on July 05, 2020:

The CEL came on my 2013 Landrover LR4 while driving. As a precaution, I tightened down the gas cap before restarting but the light remained illuminated. I took my car to my regular mechanic and he indicated that when he scanned it "18 codes were thrown" and they all pointed to a fuel injector issue that would cost me $5000 to repair. He didn't supply this code list and only mentioned two of them, one being it was "running" and proceeded to turn off the light. This vehicle has approximately 80,000 miles so I took it the dealership for a second opinion and they couldn't find anything wrong with it but suggested it was probably time to clean the injectors and the throttle body. I'm torn as to what to do. My mechanic has never stirred me wrong before but that's a huge expense and I'm not convinced. So far the light has not come back on but I haven't yet driven 100 miles.

Glenn Stok (author) from Long Island, NY on February 09, 2020:

Alan Smith - You don't need to worry about the light coming on while driving on a highway. It’s not that kind of urgency. The light refers to emissions concerns, not to mechanical concerns. When it does come on, take it to your auto mechanic to analyze the situation.

Alan Smith on February 09, 2020:

Dear Glenn,

Thank you so much for the reply. The only thing I am worried about is the underlying condition of my car. So far, I have had no problems when driving. I am a little anxious on traveling long distances. What if the engine light comes back on highways or somewhere remote? I heard it's urgent when it starts blinking. I have no knowledge on cars and its dynamics. Is it any better if I take it to a mechanic? I doubt the mechanic will see any fault without the engine light on. Any suggestions on that? By the way, I live in Japan(it does not matter, I guess). Thank you again!

Glenn Stok (author) from Long Island, NY on February 08, 2020:

Alan Smith - After your mechanic cleared the error codes to turn off the check engine light, you will no longer see any codes when you try to read them with your OBD diagnostic scanner.

If the light ever comes on again, then read the codes with your scanner and you will see what it is that’s making the light come back on.

Alan Smith on February 08, 2020:

Hi Glenn,

Thank you for the helpful post. I have a Nissan March (Micra) 2011. It began showing the Engine Check Light few months ago. Recently, I went to a dealer and he just checked it surfacely and somehow made the engine check light disappear. This was about four weeks ago. The car has run around 50-60 km after that. The light has not reappeared. I even bought an OBD reader but doubt that it will read anything as the light is not there anymore. What do you suggest? Thanks in advance!

AmyInNH on July 16, 2019:

Thank you, Glenn.

It took about 400 miles for the ODBs to go to "Ready".

Nightmare over! Well, for now.

Thanks again.

Glenn Stok (author) from Long Island, NY on July 12, 2019:

AmyInNH - Those are other issues. I’m referring to the instructions in this article if you see error code P0420 on your scanner.

AmyInNH on July 12, 2019:

Various mechanics at 4 different garages "fixed" the cause(s), gas cap, optical sensor, brake lines, etc. only to have the light return.

"...if you followed the instructions"

What instructions?

Glenn Stok (author) from Long Island, NY on July 10, 2019:

AmyInNH - Disconnecting the battery cable will cause the computer to become not ready. If there are no error codes then it should reset after driving 50 to 100 miles in individual trips. The ready status should appear if you followed the instructions.

Your story and the history of repeated costly repairs makes it sound like your mechanic is playing games with you. I would find another mechanic who is trustworthy.

AmyInNH on July 10, 2019:


-First, crazy to have to spend money to discover "Check Engine is only a loose gas cap.

-Currently, at least $3500 and over a year, and "Check Engine" relights within 4 days. Last $2600, kept it off for a mere 7 days.

-Read recently, "go for the cheapest, replace the gas cap with an OEM and reset OBD". Got it at the dealer, they said no time for reset via Scanner, just pull the battery cable. Well, now stuck in "Not Read". 1.5 months past state inspection due, and still "Not Ready"

-Asked the dealer what it takes to move it to "Ready" and they said 50 to 100 miles of driving. I said it's 250 miles driven. They said well then we don't know.

My car is captive to "Not Ready".

Am I really in the position of having a throwaway car, due to "Not Ready"?!?


Glenn Stok (author) from Long Island, NY on May 15, 2019:

Ron - You’re right that it’s not burning or leaking oil. Knock sensor problems with the Chevy Prizm were usually caused by water getting into the sensor cavity, causing corrosion of the sensor. There are two knock sensors. You need to replace the rear bank knock sensor, which is the one most likely damaged by water getting into the engine compartment.

By the way, the Prizm is a clone of the Toyota Corolla. The reason why both manufacturers denied support is because both cars were built by New United Motors Manufacturing Inc. in Fremont, CA.

Ron on May 15, 2019:

Ok. Here it goes. My check engine light is on. Code said Knock sensor. I've installed a brand new one. Well.. Apparently it was the wrong knock sensor. The car is A '99 Chevy Prizm. That was the first mistake. I bought the car for my wife. Since then we have put over $12 K into it just to keep it on the road. No one has any idea where the oil is going. I've contacted Chevy and Toyota. However, both claim they didn't make the car. It doesn't burn nor leak oil.

Glenn Stok (author) from Long Island, NY on April 24, 2019:

Jay Sim - When you disconnect the battery, the computer will lose the data. That’s why your light did not come back on. But the computer also has a “not ready” status when you disconnect the battery, and therefore you will not pass your state's emissions testing.

If the light keeps coming on, you need to know why. Since your mechanic already checked that, and told you it’s the sensor, that’s good. Replacing the sensors is a lot cheaper than the catalytic converter.

Jay Sim on April 23, 2019:

Hello Glenn really helpful post. So today check engine light came on i reset it by disconnecting the battery because i didnt have a OBD scannder. Light went away. does that mean that its a minor issue?

i had a light on few weeks ago because of the o2 Sensor but then mechanics told me it doesnt need to be fixed immediately because it is minor so i told him i will fix it next month so he just erased the code that day and my light came back on today. so is disconnecting the battery same as resetting by "erase code" that you wrote on the post? and my Car Computer system will scan the car again and engine light will be back on if there was actually a problem right?

Glenn Stok (author) from Long Island, NY on April 01, 2019:

Aurora Castillo - Cylinders can’t be replaced. They are part of the engine block. If you have cylinders that are losing compression, their valves should be checked to be sure they are working properly.

Aurora Castillo on April 01, 2019:

Hi I have a question, I have a 05 Ford expedition and it was running great I took it to get smog and it passed but the next day it was driving funny and than my check engine light went on. The guy who did my smog was rushing and not paying attention to what he was doing because he was talking to someone else that was there working on there car. Could he have done something wrong by mistake that would cause 4 of my cylinders on my passenger side to lose compassion? My husband said it keeps giving the code P0325 rich and when we had it looked at they said I need a new engine because those cylinders are reading at 150,175,175,175 and they are supposedto be at at least 200 like my other 4 are. I had new spark plugs put in not that long ago. I am just trying to figure out how one day my car was driving perfect and the next it was broken down and the only place i had went was to get smog. Also couldn't I just replace those cylinders instead of having to buy a new engine? Any information you can help me with will be much appreciated thank you.

Glenn Stok (author) from Long Island, NY on March 15, 2019:

Walid halaby - If you are referring to the service light, that’s different from the check engine light. It’s simply a reminder to get an oil change. It usually comes on after 3000 miles or so, depending on manufacturers requirements. See your vehicle’s user manual for the method of resetting it. The service station usually resets it when you get an oil change.

Walid halaby on March 15, 2019:

What the meaning of service check engine and and yesterday I turned off but today was back and light why

Glenn Stok (author) from Long Island, NY on March 10, 2019:

Janice Welch - Changing the battery will always cause the computer to read “not ready” until you’ve driven a while as I mentioned in this article.

You didn’t say what the error code was. You need to know that. Get your own code reader as the one I use. It will save you the expense of having Honda read it for you.

Janice Welch on March 10, 2019:

I bought a used 1998 Honda CRV and the guy said that he put a new battery on and the check engine light came on he checked it and said he thought it was from the battery the light went off the next day the light came back on and I took it to AutoZone and and the computer said error error do I need to take it to a dealership Honda?

Glenn Stok (author) from Long Island, NY on January 27, 2019:

W.C. - I advise you to find yourself a trustworthy machanic. The story you tell me makes it sound like they think that they found a gullible fellow. If that’s the case, they will keep milking you dry until you give up.

The only thing that was truthful was that they said the car will eventually fail. What they were referring to is that your inspection time is approaching and the car will fail the inspection. I could tell you that too. If you have any code at all registered in the computer then the state inspection will not pass.

W.C on January 27, 2019:

Hi Glenn,

I drive a Subaru Forester 2009 Turbo with 131,000 miles

I had a p0021 code on throughout last year.

I was surprised I used it throughout the whole year because around February, a mechanic had told me it was going to fail soon. However, it runs well and I drove over 17,000 miles last year.

I had the code cleared numerous times but after driving close to 40 or 50 miles, the engine light triggers again registering the same codes.

I took it to different dealerships and here are the list of repairs that have been done to repair this issue

OSV valve replacement - $345

02 sensor and Oil filter change - $325

Check for failed turbo - $500

These expenses excludes all the diagnostic fees I paid at the dealerships. It was concluded that my Turbo is fine but they are now recommending that I replace my cam gears which they estimate around $1,000. When I consider the diagnostic fees and expenses on this code, it totals about $1,700. My car failed inspection in December leaving me till the end of February to fix this problem before I can keep driving.

I am now considering getting another car if my current car is going to rack up a lot of money in repairs. Please can you advise?

Thank you

Glenn Stok (author) from Long Island, NY on January 27, 2019:

Liza - Your problem needs personal attention by an honest machanic who can physically observe your car and connect test equipment to determine the problem.

There is no way anyone can help you remotely over the internet. Of course if you get your own diagnostic tool like the one I use, you’ll have a better idea what they are telling you so you won’t be left in the dark. It's worth the small investment in my opinion.

Liza on January 26, 2019:

I have a 2007 merecedes c230 with 36,000 miles I needed a smog the check engine light was on I thought it was from the gas cap.Got the repairs fixed I had to drive the car drove 210 miles and it still says not ready.Dealer said 6 monitors were ready I need the 7th one to be ready.Im frustrated dont know what else to do.HELP!!

Glenn Stok (author) from Long Island, NY on January 18, 2019:

Rose Ackley - Try my suggestion as explained in this article.

Rose Ackley on January 18, 2019:

I have a 2014 Dodge Avenger. The check engine light has been on since last year. I had the catalyst converter replaced and just recently both O2 sensors. The check engine light was cleared, but came back on. So frustrated with this, it needs to pass inspection. I was told the catalyst converter code is permanently stuck in the cars computer.

Glenn Stok (author) from Long Island, NY on January 15, 2019:

Christina - Changing the battery does not make the check engine light come on. However, it does cause the computer to be in a "not ready" state until you have driven for a while. It's possible that in your case (you didn't mentioned what car you have) the light stays on while not ready. You may have to drive up to 100 miles before it resets.

Christina on January 15, 2019:

Hi so is it ok to drive the vehicle its weird cause i just got new battery and now the check engine light went on

Glenn Stok (author) from Long Island, NY on September 20, 2018:

Mark, Advice in this case would only be guesswork. Anything is possible. Only a visual inspection of the vehicle will provide an accurate determination of the cause of the P0730 error.

Mark D on September 20, 2018:

Hi Glenn!

I have a 2011 Jeep Grand Cherokee (104K miles). The check engine light came on while I was driving and suddenly I was unable to accelerate. I pulled over and turned the car off for a moment, and then it drove fine afterwards and the engine light cleared a day later. This has happened 3 times in the last 6 months.

The code I got was P0730: Incorrect Gear Ratio. Wondering if it's as simple as replacing a sensor or if I need to replace the trans fluid or the shift solenoids.

Any advice you can give is very appreciated!

Glenn Stok (author) from Long Island, NY on August 06, 2018:

Bruno, The answer is obvious. Your mechanic didn't read the status. He or she probably didn't think to look.

The status is always "not ready" after you clear the memory. You need to drive more than 30 miles before it registers a ready condition.

bruno on August 06, 2018:

i got inspection; at the end the mechanic was surprised to see rejection on emissions; apparently his system did not say "not ready" - is that possible; my memory had been cleared recently & i have only driven about 30 miles since; my question is why did the inspection person not see a "not ready" - once he printed the paperwork it explained the status to be "not ready"

Glenn Stok (author) from Long Island, NY on February 28, 2018:

kedarp - The only thing I can imagine is that the part is not OEM and there may be some unknown differences in the manufacturing. Sorry I can't give you a more useful answer.

kedarp on February 28, 2018:

My Nissan Maxima (2005) needed a camshaft sensor replacement which i completed. The symptoms were almost textbook like with P0340 error thrown and the car switched to 5th gear (fail safe). The problem was intermittent till it finally wasn't. So well, I got a new replacement sensor from O'Reilly which solved the P0340 but almost immediately threw a new error code P0011 which is to do with excessive timing advance. I thought the new sensor wasn't an exact match for my Maxime and changed manufacturers. I used aftermarket parts (ensuring it was meant for my NISS Maxima) as the OEM is very expensive. But depending on the manufacturer, the P0011 would come almost immediately or after a few hours/days. Finally, I got a new aftermarket from Autozone as none from O'Reilly really worked. The problem is, this Autozone sensor does not cause P0011 to be set unless the vehicle exceeds 60 mph. I have now driven the car for 200+ miles in city traffic without P0011 getting set. But the moment I hit the freeway and exceed 60 mph, P0011 gets set. This is a real pathological case and hence wanted to get your opinion on what might be the cause.

Glenn Stok (author) from Long Island, NY on January 03, 2018:

awbs - You need the device like The one I use to shut off the light. Then drive around until the computer registers a ready status again. But you need to fix all the issues, or else the light will come on again. In some cases the computer may reset the light automatically after about 100 miles if the problem was fixed. But this is not usual.

awbs on January 03, 2018:

P0301, 0302, 0303. I had the Autozone printout those codes for me after seeing check engine light on my 2000 Jeep Cherokee. I just replaced all of the spark plugs with Champion and the coil pack as well. The check engine light is still on. Are those codes supposed to disappear right away if the problem has been fixed or do I need to drive the car around for 40 to 50 miles for the OBD to recognize that the problem has been fixed? If so, what is the best driving I should do to clear the light? My main worry is I didn't fix all of the problems that is causing the check engine light to come on. I didn't ask the mechanic to clear the codes. Should I have asked him to clear the codes then drive around? I thought it was better to let the OBD figure out on its own that the problem has been fixed and the check engine light will go off.

Glenn Stok (author) from Long Island, NY on December 27, 2017:

Chris Bain - This code means you have a Ratio Imbalance of Air to Fuel. As you already know, a number of things can cause this. You need to have your mechanic check it.

Chris Bain on December 27, 2017:

Well my engine light ddoes not come on around town but went awat last week drove 200 miles light is bacķ code is P219B As there is a number of things under this code

Glenn Stok (author) from Long Island, NY on December 15, 2017:

Janice - You said you replaced the bad sensor yourself. But you didn’t reset the check engine light. You need a device like the one I mentioned in this article to reset the error codes.

Janice on December 15, 2017:

My 99 Malibu had an 02 sensor that needed replaced. The check engine light came on, and is still on after it has been driven and part replaced. We did our own work, why is it still on, and how can we reset it.

Glenn Stok (author) from Long Island, NY on July 14, 2017:

Thanks for that information, Art, That's an interesting feature to check "ready status". I wonder how many car manufactures include that? it's probably not something they would mention in the owner's guide.

Art on July 14, 2017:

Ah, I meant "guess" in a sense of I'm guessing I'll just have to put more miles on it before the monitors will read "ready", although I'm continually checking to see if they are. There's a trick I came across for my car to see if the systems read "ready." I have to turn the key to the "on" position and wait 20 seconds. If the CEL blinks, then the systems are still not ready, whereas if the light goes off immediately without blinking, the systems are ready. At which point I'll be able to complete the inspection. However, a code reader is definitely on my shopping list so I won't have to keep borrowing one from the local auto shop. Thanks again for your insight!

Glenn Stok (author) from Long Island, NY on July 13, 2017:

Art - Disconnecting the battery causes the computer to loose the status. As I mentioned in this article, you need to drive roughly 100 miles until the system resets the ready status. The fact that the check engine light is not on is a good thing. There is nothing wrong. A device like the one I use, that I mentioned in this article, will show you when all components display "ready status" again. So you can save yourself a wasted inspection by checking this first yourself.

Art on July 13, 2017:

Failed my inspection today due to the catalyst and o2 sensors both reading not ready. No check engine lights have been triggered so I'm assuming I just need to give the car time to test itself. I recently replaced a few things and disconnected the battery which could very well be the cause. However, even though both sensors are reading "not ready", this wouldn't prevent the components from sending a CEL to the dashboard in the event of a malfunction, correct? Or alternatively, would the sensors reading "not ready" indicate a potential faulty component (o2 sensor for example) and prevent a CEL from illuminating? The gist I get from your article is the former would be the case.

Glenn Stok (author) from Long Island, NY on July 06, 2017:

Ed82a - Sounds like it's clearing the codes by default. Maybe you have a faulty scanner. Check with the manufacturer about that.

Ed82a on July 06, 2017:

If whenever I plug in my scanner the engine light turns off, what could that indicate? It does not allow me to read any code because it literally turns off as soon as the scanner gets plugged in.

Glenn Stok (author) from Long Island, NY on June 23, 2017:

Izak Leen - A loose gas cap is one of the things that activates the check engine light. The code displayed will indicate if it's the gas cap or something else.

Izak Leen on June 23, 2017:

My engine light was coming on. After searching for help on Google, it tells me to check the fuel cap because it has a sensor and it's not supposed to be wet. After drying it out, the engine light disappeared. Is that normal?

Glenn Stok (author) from Long Island, NY on March 27, 2014:

ChefK - My opinion doesn't matter. You have to know who to trust or get your own scanner and see what's going on for yourself. A useful thing to do is bring your scanner with you when going to purchase a used vehicle. Check it before you purchase. Of course, that won't help in your case since you already purchased that used vehicle. But something to consider in the future.

ChefK on March 27, 2014:

Just purchased a 2003 Chevy Suburban in order to be able to haul a travel trailer. No check engine light on when we test drove it or purchased it. After having the car about a week, the light went on, We took it back to the dealer and they cleared the code. We drove about 75 miles and were able to get the car indpected successfully and thought all was well. The light then came on almost immediately after inspection. We brought the car back to the dealer. They said the car needed a new fuel pump and did ud the "favor" of only charging us for the part ($300.00) and no labor as we'd had the car such a short time. About a week later the light came back on. I was afraid I might not get home from work. Stopped at Auto Zone to get the codes read and they asked if I had just purchased this vehicle and whereas 27 different codes came up (some twice). A this point I've had enough. Brought the car back and they replace mass air flow and reset light. I have no idea what to do as I really think we've been fleeced by this dealer and we've decided if the light goes back on-since they've told us all they can do is guess at what the problem might be, is to take it to our old mechanic (who has always been shown to be honest) and bite the bullet and pay for whatever else might need to be repaired. Any suggestions? Also, I'd really like to know if any of the codes that show up when scanned by Auto Zone could be old codes for issues that have already been resolved as this is what the dealer is trying to tell us.

Glenn Stok (author) from Long Island, NY on March 21, 2014:

Dianne - Seems that you may have a short. But it can only be determined by investigation by an honest mechanic. Meanwhile, if there are any codes in the computer, one of these "code scanners" will show you that.

Dianne on March 21, 2014:

My check engine light will come on and go off immediately, only when making a right turn. I have an '04 Expedition, I've taken it to Autozone, and they say they can't check for codes unless the light is on, but it never stays on. Any ideas what this may be?

Glenn Stok (author) from Long Island, NY on March 10, 2014:

D R Terrill - It's obvious.

D R Terrill on March 08, 2014:

I took my 97 Merc. G. P. to an independent garage for an oil change and state inspection. The mechanic stated that the test showed both cat. converter failed. He said engine light was mayby on the verge of turning on. I said I am not going to spend the large sum of money on this old car. They offered to buy it at 1/3 of it's value. I drove it home and a few days after, got to checking the oil and found that they over filled oil by 4 quarts and my old filter was not changed. I feel like I was set up for that cat. converter failure. What do you think?

Glenn Stok (author) from Long Island, NY on March 07, 2014:

John D - Your problem is not related to the subject of this article, but it sounds like your transmission needs to be checked. Bring your car into a mechanic.

John D on March 07, 2014:

Hi, I have a 1992 Ford Thunderbird lx 3.8 automatic. Sometimes when I press on the gas it's like it's in neutral. And if on a hill will roll back. It's like I have to rev it up for it to catch then start going?

Glenn Stok (author) from Long Island, NY on February 19, 2014:

Tim, Since no codes came up, it's not related to any of those issues. You need to have your mechanic look at it to determine the problem.

tim on February 19, 2014:

Recently we have been having some very cold and snowy here in Ohio. I have a 2008 avenger and a few days ago the electronic throttle control indicator began coming on. When this occurs it won't allow me to go over 35-40 mph and it stutters. After a few seconds I let off the gas and allow my mph and rpm to drop and it will be okay for a little, but my rpm remain kind of high (around 3). When I had the codes scanned nothing came up. Anyone know what this could be and how donk fix it? Should I have a shop update my computer system?

Tech on February 19, 2014:

Jen, if you have your mechanic replace the oxygen sensor(s) the proper permanent solution is to find and stick with a qualified technician you can trust. He can DIAGNOSE the trouble code(s) properly. There are tests that can be done to determine whether a converter and/or oxygen sensors are the problem. Some vehicles have updated software for situations like these where parameters are re-defined. A good technician can tell you this. A code read/quick scan may not reveal all stored codes like a sophisticated scantool will. A technicians scantool will also provide valuable freeze-frame data which will help with the diagnosis. No good shop wants to put a part in only to have a customer return upset when the light returns--meaning an expensive catalytic converter doesn't do a shop any good either when it doesn't fix a problem. Nothing wrong with a second opinion, just like the doctor, but anyone good will charge for their time. Build rapport and trust, technicians have to trust their customers like you have to trust them. Good luck.

Glenn Stok (author) from Long Island, NY on February 14, 2014:

Jen, You have an honest mechanic. Code p0430 does refer to the catalytic converter. But as I mentioned in this article, it might just be the sensor that's bad. It's cheaper to try replacing the oxygen sensor first. Another simple problem could be an exhaust leak.

Jen on February 14, 2014:

Hi Glenn!

I have a 2006 Jeep Grand Cherokee Laredo with the CEL on. We had it reset, came back on....Our mechanix ran a code p0430 but said he didn't think it was the catalytic converter. The car runs great, accelerates great, no issues with gas mileage and has never stalled out. Would you first suggest replacing the 02 sensors to see if thats a fix?

Glenn Stok (author) from Long Island, NY on February 04, 2014:

Veronica - Code P0700 only indicates generally a fault in the transmission. You need to get a reading of the exact code from the transmission module. Have your mechanic check further into detail to determine the exact problem with your transmission.

Veronica on February 04, 2014:

I have a 2005 Chrysler 300. I had a diagnostic test performed and the codes that registered were:p0700 and p0562. Does the p0700 code mean I need to replace the transmission?

Glenn Stok (author) from Long Island, NY on January 25, 2014:

Will - I don't know why your code scanner fails to clear. You'll have to ask your mechanic to check on that. But here are the meanings of the codes you have:

P0171 Bank one running too lean. Too much air or too little fuel.

P0700 Transmission Control System Malfunction

P0562 Low System Voltage

Will on January 24, 2014:

Thanks for your informative article. I have a scanner tool that I have used to clear codes to see if the codes recurr but presently when I attempt to clear codes the scanner tool displays 'fail' and the codes remain. Why is that? My MIL is not on but my codes are p0171 and I have two recent pending codes p0700 and p0562 (we had a very cold week and I think the last two codes appeared because of a cold weather start).

Glenn Stok (author) from Long Island, NY on January 22, 2014:

Lori - The 2006 Chrysler 300 uses an electronic throttle control. The throttle control warning light (a yellow icon) comes on to indicate a problem with the electronic throttle system. Is this light on? Did your mechanic read the code? If you know the code, you can look it up yourself. Is your engine a diesel? You may have a faulty swirl pot actuator or it may be getting jammed. Ask your mechanic about that and leave another comment on the results.

Lori on January 22, 2014:

My 2006 Chrysler 300 loses throttle power. Changed throttle body - twice! Also changed EGR valve and finally the battery; all to no avail. 4 trips to 2 different dealers (240+ miles away) and they have no answer, other than possible PCM failure. Any suggestions? I'm ready to 'accidentally' roll this off a cliff ;)

Glenn Stok (author) from Long Island, NY on January 13, 2014:

Isaac - Sounds like your mechanic just wanted to make some money for other things that may or may not have been necessary. You need to have your mechanic tell you what the code was and ask to repair JUST that issue. Once you now the code, look it up and you'll know what is really wrong. Or get your own code reader like the one I use and read the code yourself.

Isaac Monyela From Palm-Ridge,Johannesburg,South Africa on January 13, 2014:

I'm driving an Opel Corsa Utility Van,I took it to a mechanic for the engine light and he replaced the brake light switch,plugs,petrol pump,oil, and after the light still comes on.please help?

Glenn Stok (author) from Long Island, NY on December 05, 2013:

GinaB - I don't know if it's common or not with the Toyota, but the problem might just be a bad connection with the wiring harness connecting the knock sensor. It may be worthwhile to try replacing the wiring harness, which costs about $160. Ask you mechanic about that. It's labor intensive to replace the sensor and can total $600.

GinaB on December 04, 2013:

Glenn, I have a 2007 Toyota Camry & my check engine light comes on every now & then. When it comes on though, when I press on the gas to accelerate, it goes very slow. I can have my foot all the way down on the pedal & slowly reach 20mph...then it drives fine once I pass 20mph (2nd gear). I recently went to the mechanic & when they connected it to the scanner he stated it was the knock sensor. I've been shopping are for different pricing to have it repaired however one mechanic asked if I was sure that it was the knock sensor because that is not common for a Toyota. Any advise would be greatly appreciated!

Glenn Stok (author) from Long Island, NY on November 25, 2013:

garymitchell - Thank you for your comment. However, it cannot be approved for posting since you included a self-promotional link that is against HubPages' terms of service.

John teens on October 01, 2013:

I retired after 43 years of mechanic.Abont 20 years auto and 23 in heavy duty trucks. best article on codes I've ever read . I've had extensive training in electronics of all aspects of automotive computers being employed by king county in wa state. Great article.

Glenn Stok (author) from Long Island, NY on August 30, 2012:

mjkearn, A lot of people get fooled by the check engine light and end up spending hundreds of dollars in unnecessary repair bills. It means a lot to me that a knowledgeable car mechanic such as you has given his seal of approval on what I wrote. Thanks for such a nice comment, for the vote up and for following me too.

mjkearn on August 30, 2012:

Hi Glenn, this is the best written, most informative explanation of the check engine light and scanners that I have ever seen. Couldn't have explained it any better.

Enjoyed the quiz and have to confess to not getting 100%. May need to retrain I think. I thought some early systems had cumulative memory and recorded all history and resets, but then again I'm "nuts and bolts" more than electronics.

Thoroughly enjoyed this hub and thanks for writing. Voted up and the rest,


serebren on June 30, 2012:

Nice article Glenn, but I have a few things to add.

Some auto parts stores (Autozone, Advance) will read and interpret the OBDII codes for free. If you already have a code reader, you may be able to go to their website and look up the codes. Most mechanics charge for code reads, but they also have a more sophisticated reader that will do in-depth scans.

Most common code? P0455, usually caused by the gas cap. Before going straight to the cap, however, ask this question: "Did the person who filled the car with gas stop when the pump stopped, or did they keep going after that?" One of the most common problems with the EVAP is that overfilling the gas tank will swamp the EVAP system with liquid gas - it is made to recover vapor, and prevent it from escaping and polluting the atmosphere. Two parts are negatively affected by flooding - the solenoid valve and the EVAP canister. Each usually runs about $200 - $400, and you usually have to drop the fuel tank to get to them and replace them.

A third common problem is the EGR. Often, the tube to this part is clogged with carbon, preventing operation of the EGR valve (which is not bad). Clogging is more prevalent on some cars than on others, and depends more on the tube - tight bends tend to cause this problem, and both sides (at the cylinder head and near the throttle body) need to be checked.

Glenn Stok (author) from Long Island, NY on June 05, 2012:

Did you buy new rims? Some are not compatible with the tire pressure sensor as I discuss in this article. I never heard of a reset, but check your cars's owner's manual about that. If your car has that feature it should be mentioned in the manual.

victoria from Hamilton On. on June 05, 2012:

Hi! Thanks for the great info.!

My trouble is my check tire pressure light/message.

My neighbour filled my tires but it still came on and beeped.

I have driven it about 50 kilometres since then.

another neighbour said to turn the key to on and hit the reset button twice.

So do you think that would help?I am newly widowed and do not have a great deal of experience with cars.

Thanks for a great useful hub.

Glenn Stok (author) from Long Island, NY on April 17, 2012:

Kristina, Since you had a new catalytic converter put in and the light still keeps coming back on with the same code, that unfortunately means that it is a bad sensor or that the shop gave you a defective catalytic converter.

Did you buy a rebuilt one or a new one? Did they replace the sensor too, just to be sure. The ready status occurs about 100 miles after you reset it.

Maybe you can squeeze in the inspection after you see that your scanner shows "ready" and before the light comes back on. But go back to your mechanic and as the above questions in any case. Good luck.

Kristina on April 17, 2012:

Hi Glenn! Thanks for the informative article. I have a 2002 Monte Carlo SS and my CEL keeps coming back on with the P0420 code. I borrowed one of the Actron diagnostic scanners you mentioned from a friend, and have cleared the code 3 or 4 times already, but it keeps coming back on.

The thing is, I had my catalytic converter replaced in October! The code first came back on not even a week after the replacement, and has done so every time I've cleared it. I would be fine with it, but I still have to pass my VEIP test for MD state inspections and the last time I went in I got a not pass/not fail, which I'm assuming is because of the Not Ready status you mentioned from clearing the code myself.

Is there a reason the same code would come back immediately after I had the issue resolved?


Glenn Stok (author) from Long Island, NY on April 16, 2012:

Jacob Roberts, I discussed that in my article. The computer gets reset back to ready status in less than 100 miles. Not 500. But your mechanic should have shut off the light. If you paid $350, ask him or her to do that. Or get your own code scanner such as the one above for $20 and turn off the light yourself. Then drive about 100 miles and check it again for ready status. More details are in this article.

Jacob Roberts on April 16, 2012:

After $350 repair (replacing the EGR Valve). My Check Engine light was out for only about 5 miles, But it's back on again. called back the mechanic and said that I have to drive the car 500 miles for the engine light to go out, thats how long it takes for the computer to reset. I need to smog this car. This is unacceptable. That's mean I have to drive out of state. FIVE HUDRED MILES? really? is that true?

Glenn Stok (author) from Long Island, NY on April 11, 2012:

Jan, Any business has a right to charge for service. Connecting their code reader to your car and reading the code is considered a service. But you can do it yourself if you have your own code scanner. If they told you the cost of the service and you agreed to have them do it, then it is acceptable.

Jan on April 10, 2012:

Hi again,

Yes, I see that now - but I didn't find your fabulous article until I was Googling to find out if I was ripped off or not (and if I should complain and ask for a refund or a credit).

So, is it legitimate to charge a code reading fee? Or was I "hosed?"



Glenn Stok (author) from Long Island, NY on April 10, 2012:

Jan, For less than that $50 you paid for a one-time reading, you could have purchased your own code reader such as those shown in this article.

Jan on April 10, 2012:

Hi Glenn,

Thanks for all the great information. I just had my Trailblazer inspected (the Check Engine light was on), and had to have a thermostat and sensor replaced. When all was said and done it was $398, and that included the $37 inspection fee (which we expected), and a $50 "Code Reading Fee" for hooking it up to the computer - which was totally unexpected. Is that legitimate or is this guy a thief?

Thank you,


Glenn Stok (author) from Long Island, NY on April 09, 2012:

Daria, I would trust that last mechanic who you mentioned. It's true that if the light doesn't come back on then you should be fine.

daria on April 09, 2012:

The dealer said I had code P0711, transmission fluid temp sensor performance out of range, tested sensor and verified failure. They reset it (back in December). The light has been off since then. The CEL came back on Saturday, and I put gas in today (Monday), and it went out. How do I know that it isn't just a gas cap being loose and not something with the transmission? I am looking at repair cost of over $500 if the dealer is correct, but how do I know they are being truthful? It has been on one other time, and it seems to happen around feuling times...so I am thinking it's just a loose gas cap....what do you suggest I do? One mechanic (not at the dealer)told me if the light didn't come back on, or I didn't notice any issues, not to worry about it. Who do I trust? Thank you!

Glenn Stok (author) from Long Island, NY on April 05, 2012:

Bob, P0601 means the computer failed a self test. This can also be caused by bad wires running to it. Or grounds on a sensor may be bad if they became rusty. If you do buy one, make sure it includes a warranty. Rebuilt PCM's are not always reliable.

Bob in Dallas on April 05, 2012:

Hello Glenn. I have a 2002 Dodge 1500 2wd Pickup with a 5.9 gas engine. The check engine light is on and the code reads P0601 which indicates a ECM failure. The truck runs fine, however I need to get and inspection soon. I can order a new rebuilt ECM from AutoZone and have read the simple installation instructions. My concern is spending $250 and it not fixing the problem. Any suggestions or guidance?

Glenn Stok (author) from Long Island, NY on March 17, 2012:

Chee, That's the idea. But I don't know if they would be willing to clear your codes. The best thing is to do it yourself. The cheapest $20 scanner like the one listed above is just as good and will also allow you to clear your codes.

Chee on March 17, 2012:

Hi I have a 1998 Toyota Avalon and the check engine light just came on. You mentioned in the video to ask the service department if they can scan the code for free. Could I ask them to erase the code as well? If they erase the code, and then I continue to drive without the light coming on then maybe I can avoid a big repair. If the light does come back on, then I know that I should take it back and can invest in a more serious repair that is needed. Thanks.

Glenn Stok (author) from Long Island, NY on March 12, 2012:


Have your mechanic clear the code again, or buy your own scanner like the ones listed in this article and do it yourself. If the light doesn't come back on so quick then maybe it's just a bad sensor.

You can also try putting high test gas in your tank (when empty, so you fill it up) and drive with high test gas just for the one time. This has been known to clear out the catalytic converter at times, and the problems will be resolved. It worked for me, and the light stayed off now for two years.

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