How to 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 whether 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 a number of 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 to expect 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 near the end.
- 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 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 from the computer and watch to see if the check-engine light comes back on. Your car’s diagnostic codes can be read with a simple code scanner. It plugs into a special plug that you can find under the dash.
These scanners can also be used to reset your car’s computer by clearing any saved trouble codes.
Since I have a background in computer science, I am aware of an important issue: The only way the car's computer can know if a unit is failing is by a sensor reporting back to the computer. Each sensor specifies a particular code. When the computer has a code stored, the check engine light is lit to let you know. The computer registers a specific trouble code to indicate 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 or Erroneously Trip
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 bad 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. This 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 what the computer is saying about the check engine light 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 really good prices and a large 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 cars since 1996 use the OBD II codes. The more expensive Code Scanners can read the older OBD I codes as well. That is simply not necessary to have, unless you have a really old car.
Why It's Worth Getting an OBD Code Scanner
Replacing the catalytic converter can be expensive for parts and labor combined. If you are scheduled for your annual State Inspection then you will be forced to pay for the repair in order 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 erroneously trip 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 catalytic converter. In many cases it's just a bad sensor and you wouldn't know that unless you have the ability to 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 OBDII Vehicles just like the one I use. You can flip through a menu of options on the display screen 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, or visa versa.
This scanner doesn't require me to look up the codes in a table either. 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.
This 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 common 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 There Really Is 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 over time.
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 code in simple English. It's good to know this before erasing the codes. Then I chose the option to erase the codes from the computer.
But you're not done yet! In order 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.
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 doing 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.
This applies if you were to take your car in for an inspection. Simple answer, you will not pass inspection if the light is not off. This is accomplished by clearing the codes from the computer, but in order to pass inspection, the computer 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 car at that time.
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.
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 in order to have many more features that replicate what can be done by service mechanics. I don't find any need to 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 for you.
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 bellow 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 method is indirect 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 to 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 which is intermittent, then you may notice the light goes off after some time. But it may come back on again once is 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 computer. If the Catalyst System Efficiency is below 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 let's 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 complete 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 ready 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, 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 to wait.
- 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 simply 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 statistics© 2009 Glenn Stok
Reader Comments (Most Recent First)
Hi Glenn,
Thank you for a great article, I learnt a lot.
My engine light lit up three days ago. I had the mechanic take a look this morning and they are telling me I need a new catalytic converter. When I asked whether it could be a sensor issue and whether they could test that (sensor versus actual CC aging), they said it was the CC, not a sensor problem.
My first question is: can they really tell that, this fast? Or are they possibly trying to get me to buy an expensive piece...
I'm about to buy a scanner like the one you describe in your article, to try your way (see if the code comes up then erase the computer and drive 100 miles and see if the light comes back). There is something I don't understand: if the sensor is the problem, and all I do is erase the computer (without replacing the sensor), why does the light coming back confirm it's a real problem versus a sensor problem? The sensor, if not replaced, would still be at fault after rebooting.
Thanks a lot for your help.
I am at my wits end with my 2004 Jeep Grand Cherokee. I go to a reliable garage, have used them for years, but they cannot seem to find out now why my Jeep is not running right. It all started when the check engine light came on. In the past I've bought a new gas cap and that solved the problem. This time that didn't work. A sensor was replaced and ever since then I've had the Jeep back in the shop at least 4 times since this was done. The problem is that the engine and the transmission, which I had rebuilt last year, are not communicating properly. The engine and transmission were working together just fine when the check engine light was on. Since that sensor was replaced, on the straightaway the transmission works great. However, when I turn to either the left or the right, the transmission doesn't shift smoothly and almost stalls out. If I turn at 15-20 mph the transmission shifts okay. However, if I turn faster than 20 mph, the transmission acts like it doesn't want to shift. For the past two days when I've backed out of my garage, my Jeep shudders when in reverse and once I put the transmission in drive, the transmission acts like it is going to stall out. When it does shift, it takes a few seconds to get in gear and then lurches forward. Help!! Any advice would be appreciated. Thanks so much.
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.
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.
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
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.
our car had a new engine put in the check engine light has stayed on even though everything is fine. will it pass inspection of will the check engine light cause a fail
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!
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.
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.
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?
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.
Very informative Glen. Novice for newer cars, limited to plugs, lights, brakes, etc on older ones. Just had a code read at local Auto Zone and O'Reileys, both agreed on Cylender 2 misfire. Am fairly confident I can fix, even being recently disabled. Your web site is great. Will be buying a Diagnostic Code Scanner per your suggestion. Thanks
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?
I brought my van in to the dealership to have the warranted catalytic converter replaced. Two private mechanics had run diagnostics and I already know that is what was needed. The service manager called to tell me that I also needed an EGR switch to the tune of $486 because he "thought" that is the reason the CC failed. I told him I couldn't afford the EGR valve right now to just do the warranted work. He refused and told me he was not putting on the new CC unless I replaced the EGR. Then he went on to tell me "you have no choice." To which I laughed and said oh yes I do. Well to make a long story short, I realized after further conversation that he was scamming me so I told him I would be there to pick up my van and bring it to another dealership who agreed to replace the CC no questions asked and no intention of extorting money from me for an unwarranted part. Problem is he still charged me the diagnostics charge when in fact he was the one who did not keep his end of the contract, by threatening and trying to force repairs I did not want. So in short, has anyone had a service manager try to extort money for repairs? I will get my money back from this dealership. When I get finished they will wish they had never met me, let alone try to scam me. Seems dealerships have sunken to a new low.
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?
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?
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.
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?
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?
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).
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 ;)
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, 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!
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.
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.
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.
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!
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?
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
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
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!
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?
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.
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?
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.
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?
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.
Hi Glen, I bought a used 2000 Nissan with 200,000 miles on it and it is almost time to be inspected but have had trouble with various lights coming on, Check engine, service engine soon, and the abs light all coming on at various times over the last few months. Since I am not having any engine trouble and the car runs fine, do I have the computer changed or just the sensors? By the way all of the lights are now off but I am afraid to try and get it inspected until I solve the light issue.
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
Hi Glen,
I had a P0455 & P1448 diagnostic trouble code come up when I went for my NYS inspection earlier today (Failed both the safety & emissions part of the test). Like you said earlier in an earlier post P0455 could be a faulty gas cap, do you suggest just buying a new one & trying it on? Perhaps the threads on my gas cap have worn out. I'm looking at at least $1700 for repairs from the mechanic, do you think that's worth it getting all that repair work done for a 2000 sentra?
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. :-)
Hi. Came across this site while looking for information. Very interesting read. Perhaps you can help with my concern. I purchased a vgate reader to turn off the warning lite. I did this with the engine running. Then I read the instructions that said don't clear the codes wit the engine running. Whoops. Have I done any harm?
I was hoping it was something simple.....
Thank you Glen for quick response
99000 miles and this is the first check engine light.
The engine stuttered a bit and could not accelerate but it would respond to low pedal acceleration
(Enough to get home). Turned off motor and restarted .Engine responded normally with acceleration but engine light is still on. Next morning engine light is on but the motor response is normal. Went to lunch and no engine light. Can I assume that this is an intermittent sensor issue and not the something else?
Hi Glenn, thanks for the advice. I think you are right. I tried to use #93 gas to help clean the coke on the sensor one week ago. It is said this is one of the helpful ways to get rid of engine light problem. After running for about 400 miles, the light went off today. I'll try to replace O2 sensor as you suggested. Which sensor do I need to change? There are four, I think.
Again, thank you for sharing, very helpful work!
Merry Christmas!
Hi Glenn, thanks for sharing your knowledge!
I have a HONDA Odyssey 2005, 130K. engine light was on from two moths ago. When I tighten the gas gap, it went off after one week. It happened for a few times. Two weeks ago, my car passed the emission test just after the light went off. Last week, the light was on again. The code I got is P0420. If you think the cat converter needs to be replaced, how come it passed the emission test two weeks ago?
Please advise, maybe I need to pay $1000 for the cat. converter.
Happy holiday to you!
I have an 06 Subaru. The check engine light came on and I took it to a subaru dealer. They claimed they tested the flow of the cat. converter and that it was the issue. I didn't get it fixed because they wanted over $1000. It went on and off radomly in the months since then. But I recently changed the battery which must of reset it and it hasn't come on since then. I've driven the car a few thousand miles without the light coming on. Do you think this is a sensor problem?
Hey Glenn, thanks for the reply. I took my car in today cause my dad supposedly heard a weird noise. turns out it was the coil on the first cylinder that was bad....when I asked the adviser to explain further he was just like...oh it's the coil, your car is fine now.So yea..supposedly its the coil problem
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