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

As a technical writer with a science and engineering background, I solve technological problems to save money on car repairs.

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 they are 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.

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

  • Understand what your check engine light means.
  • How to read your car’s diagnostic codes.
  • Tell whether there is a real issue with your engine or merely a faulty sensor that triggered the light.
  • How to clear the light so that you can pass an inspection.
  • Understanding the "Check Tire Pressure" light.
  • Finally, I conclude with a section with common questions readers have been asking and my answers.

Understanding Your Car's Diagnostic Codes

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 and you can avoid expensive car repair bills.

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.

What Does it Mean if the Light Comes Back On After a Repair?

A reoccurring OBD code after a repair might mean that it's not the replaced part that was bad. If you had an expensive part replaced, ask the mechanic if they tested the sensors or if they examined the vehicle for other causes of the error condition.

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If you had only the sensors replaced, the part that the OBD code refers to is most likely malfunctioning and needs to be replaced. However, a good mechanic will also check the wiring to the part in question. Damaged wires can also contribute to a misleading error condition.

Choosing a Diagnostic Code Scanner

Often when I watched a mechanic plug in their code scanner under the dash to read the engine trouble codes, I thought how useful it would be having my own. I’d know why the check engine light is on before going to the repair shop. So, I decided to spend a little money on my very own Diagnostic Code Scanner.

I 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 diagnostic scanner for OBD-II uses a 16-Pin port located under the dash, usually on the driver’s side.

What if I Have an Older Vehicle?

If you have an older vehicle, you'll need a diagnostic tool for OBD-I diagnostic codes used on GM vehicles from 1982 to 1995.

OBD-I uses a 12-Pin connector, and its port was usually under the hood in older vehicles. So you would also need an extension cord so you can sit in the driver's seat to carry out the required tests while using the diagnostic tool.

A Code Scanner Could Save You From an Expensive Repair

Replacing the catalytic converter can be one of the most expensive repairs 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 the emissions test.

You may be able to avoid replacing the catalytic converter. In many cases, it's just an intermittent problem with a sensor, and you wouldn't know that unless you 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, but they all show you the standard problem codes and have a function that allows clearing and resetting the computer. Most good scanners will guide you through the process on its screen.

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 component 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.

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.

Common Questions About the Check Engine Light

These are common questions people have been asking, along with my answers. Questions & Answers about specific OBD codes appear farther down.

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.

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.

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 after having driven over 73,000 km so far, can you help?

Answer: That code refers to the Camshaft Position. The VVT (variable valve timing) or VCT (variable camshaft timing) components give the error and need to be checked by a mechanic.

Question: Will the check engine light eventually reset itself after I change a faulty sensor?

Answer: The OBD diagnostic system will eventually reset automatically if the problem is no longer detected. However, it could take up to over 100 miles of driving. Of course, you can always clear the code yourself with a diagnostic scanner.

If it is only a faulty sensor, the check engine light should not come back on.

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

Answer: The P0300 OBD code indicates “Random Misfire Detected.” That means not all the cylinders are firing correctly.

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

In both cases, it's best to have your mechanic check your vehicle to determine if repairs are required.

Question: I'm getting code P0340, which indicates a sensor malfunction for the camshaft position. I changed the crankshaft position sensor, but that didn’t help. What else should I do?

Answer: Besides the Camshaft Position Sensor, the P0340 code could also indicate problems with the wiring between the sensor and PCM. Or even a bad PCM.

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: 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 you drive close to 100 miles. However, the OBD error code will reappear and the check engine light will come back on if the original error is detected again.

Question: I seem to have problems with my transmission, and I have OBD code P0700. However, after getting the code, the check engine light went off, what should I do?

Answer: Code P0700 means "Transmission Control System Malfunction." That could be caused by various problems with your transmission or something related to it. For example, if you have difficulty shifting gears or your engine tends to stall, it can be due to this issue.

Since the light went out, it could be a minor issue. But you need to have a mechanic check it out to be sure.

Find a reputable transmission specialist by asking friends for references. The problem is that a dishonest mechanic might insist you need a new transmission when you have code P0700, but you may not necessarily need to replace it. This is because that code only indicates a fault in the transmission. A good mechanic will get a reading from the transmission module to determine the exact problem.

Question: I have error code P0171, and my car is running rough and wasting gas, what's causing this?

Answer: If your car is running rough when you have this code, it could mean there is too much gasoline in the air-fuel ratio. The PCM detects that the system is running too lean (Too much air or too little fuel. The result could be an engine running rough.

You might also notice poor gas mileage. The PCM will try to adjust for that, but it would be helpful to have a mechanic diagnose the cause of the problem.

Question: My check engine light is on with OBD error code P0137 and P0136, what does this mean?

Answer: P0136 and P0137 refer to the downstream oxygen sensor (O2 for bank 1, Sensor 2). There are two O2 sensors, one before and one after each catalytic converter. They measure the amount of oxygen present in your exhaust to determine if the catalytic converter is functioning properly.

Since you have both these codes, your bank 1, sensor 2 is not showing a voltage above 0.21 volts, indicating that there is excessive oxygen in the exhaust. In other words, the exhaust is too lean.

Your mechanic should test the voltage and ground on the sensor to determine the problem.

© 2009 Glenn Stok

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