Find Out Why 'Check Engine Light' Is On, and Avoid Costly Repairs
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
I originally purchased an older version Diagnostic Code Scanner, but there are newer ones available now in auto parts stores and on Amazon, such as this one. This thing works great! It shows On-Screen Definitions for OBD-II vehicles, just like the one I use.
You can flip through a menu of options on the display to read the computer for any pending problem codes, read the status of the individual sensors, and even determine if the check engine light is on when it should be off.
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.
Here Is a Useful Video About Reading Trouble Codes
How to Read Your Code Scanner
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.
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.
- 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.
- 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
|
|---|---|
READY
| If the check engine light did not come back on, the system is working properly and your car will pass emissions tests.
|
NOT READY
| Your car has not been driven enough after resetting the codes. You need to drive until the system indicates a READY condition.
|
NOT APPLICABLE
| 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 
Tire Pressure Monitoring System (TPMS)
Understanding the Check Tire Pressure 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, known as 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.
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:
- One way is indirect by using the ABS/wheel-speed sensors (wheels spin at different speeds based on their inflation).
- 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. 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
view quiz statisticsThis 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
The code "P0011" has appeared in my Nissan Micra 2012. It has 73000 km on it so far. Can you help?
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.
Helpful 31What do the OBD codes P0300 and P0339 mean?
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.
Helpful 21Will the check engine light go off automatically if you change the faulty sensor?
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.
Helpful 21I'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?
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.
Helpful 18- Helpful 1
© 2009 Glenn Stok
Related
Reader Comments (Most Recent First)
Thank you, Glenn.
It took about 400 miles for the ODBs to go to "Ready".
Nightmare over! Well, for now.
Thanks again.
- AUTHOR
AmyInNH - Those are other issues. I’m referring to the instructions in this article if you see error code P0420 on your scanner.
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?
- AUTHOR
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.
Painful!
-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"?!?
Insane.
- AUTHOR
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.
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.
- AUTHOR
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.
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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?
- AUTHOR
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.
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.
- AUTHOR
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.
What the meaning of service check engine and and yesterday I turned off but today was back and light why
- AUTHOR
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.
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?
- AUTHOR
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.
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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
- AUTHOR
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.
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!!
- AUTHOR
Rose Ackley - Try my suggestion as explained in this article.
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.
- AUTHOR
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.
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
- AUTHOR
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.
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!
- AUTHOR
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.
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"
- AUTHOR
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.
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.
- AUTHOR
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.
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.
- AUTHOR
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.
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
- AUTHOR
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.
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.
- AUTHOR
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.
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!
- AUTHOR
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.
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.
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Ed82a - Sounds like it's clearing the codes by default. Maybe you have a faulty scanner. Check with the manufacturer about that.
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.
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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.
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?
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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.
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.
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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.
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?
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D R Terrill - It's obvious.
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?
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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.
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?
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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.
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?
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.
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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.
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?
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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.
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?
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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
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).
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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.
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 ;)
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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.
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?
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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.
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!
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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.
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.
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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.
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,
MJ.
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.
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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.
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.
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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.
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?
Thanks!
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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.
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?
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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.
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?"
Thanks,
Jan
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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.
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,
Jan
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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.
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!
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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.
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?
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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.
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.
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Mary,
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.
Hi Glenn,
I am a college student and I graduate in May. My Pontiac Grand Am's service engine light came on about two months ago. I brought it in to the mechanic and he said it was a 420 code. Now he reset the service engine soon light and it just came back on. Can I make it to May without fixing the car or should I spend the $700 plus to fix it?
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Lindsay, This is exactly what I was saying at the beginning of this article. You never needed to replace either catalytic converter.
Since your light is still coming on, and assuming the mechanic told you his or her scanner showed it was a code for the catalytic converter, then it is obvious that it was only a sensor that was bad, as I explained in this article.
You can confirm the codes yourself if you get your own $20 scanner such as the one I listed above. And with that scanner you can clear the codes and see if the light comes back on. A lot cheaper than paying for two new catalytic converters.
Hello. A month ago I had an error code for the right catalytic converter in my 2005 dodge stratus. I had that fixed, then three weeks later I had another error code for the left catalytic converter. I have had them both replaced and the engine light came on again 10 miles down the road. I brought it in and they said they forgot to reprogram it and tell it they put a new catalytic converter in. So I thought I was done with the deal, then I left and it came back on! What the heck is going on? I only have 40,000 miles on my car.
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Amanda, It should have become ready after driving 100 miles. You may need to do that in three shorter driving sessions. Not all in one trip. If that doesn't do it, you may have some other problem. Your scanner will tell you which status is not ready.
My car had a code come up that basically said for me to change the gas cap. I changed it and a new code came up. P0171. Something about system too lean on bank 1. We cleared the light and have driven 200+ miles. I use my car every day to go to work & school. We went to get the car smogged and the tech said that it didn't pass or fail. We have access to a machine to check it and it still isn't ready. What do you think is wrong? When will it be ready again?
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LoLa, It will not damage the engine but the cat. conv. may overheat. So be careful about parking over leaves, etc. Just in case, you should try to find someone you trust to check it anyway.
Code PO420 has come up on our 2000 Astro Van. Had the Cat. Coverter replaced about 3 years ago but shop has now gone out of business. We are in Mexico and will return to U.S. in a few months but it's a 2000 mile drive home. Will driving with the PO420 code damage the engine? I have no access to a Chevy dealer in our small village.
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Rose, Thank you for asking your question. It's a very important addition to this article, as it explains what many people get into.
If your light is not coming back on after they cleared the codes from your computer, that indicates that your catalytic converter is okay.
These people sound like crooks to me for two reasons...
(1) They told you that you need to replace BOTH the catalytic converter AND the sensors. That implies that they already suspected that ONLY the sensors are the problem. And if they didn't replace them, then you would still have a problem, and they needed to hide that from you.
(2) The biggest give-away that they are crooks is that said your car will blow up. That is just silly and they think you are dumb and gullible, which you are neither. Stay away from that mechanic!
Now that you have your own code scanner, you can clear the codes yourself and watch to see if the light ever comes back on. If it is intermittent then it probably is only a sensor. Either way, emission problems never cause a car to blow up.
I am glad I found your website. I took my Nissan to the dealership after the Service Engine Soon light came on and I was told that I need a catalytic converter and 02 sensors which will cost $1500. I told them I would think about it. I had to $105 for the diagnostic test. Questions: Should I have gotten some type of printout from the diagnostic test? The engine light doesn't come on now. I guess the dealership turned it off. So after reading your comments that it is best to turn it off and see if it comes back on and then you know for sure you have a problem. Now I can wait and see. The dealer told me that my car might blow up when I drive it off the lot. Thanks so much for a great website. I have put your site on my favorites list.
Yea I was actually thinking the samething, it wont harm to just try it.
Well thanks a lot Glenn you have been very helpful
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Von, this does not sound like your cat converter. It sounds like your gas cap is not on tight. Simple solution. Check it.
Thank you for the article. Service engine soon came on in my 2001 Blazer so I took it to Auto Zone. Their scanner would not work on my vehicle. They said it might be a fuse. I remembered that I had accidentally started the engine with the gas cap off so I unhooked the battery to reset the computer. Drove it several times for total of 100 or so miles and the light is back on. I'm female by the way and don't know a lot about cars. Has anyone ever had a scanner not read out? My gas gauge has been acting weird too. I did purchase my own scanner from the Ebay link on this page and hope to eventually get a reading and avoid that costly catalytic converter repair. For $30.00 it was hard to pass up. :-)
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