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)
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Marine30605 - It's not easy to determine if it's just a sensor problem, but that is indeed a cheaper item to replace if you want to try that first. A stuck sensor will give the same code as if the CC was actually at fault.
It's worth trying to clean the crud out of the CC first with the method of using high-octane gas as I described. This has solved the problem for me and many of my friends who had the same issue.
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.
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Isabelle38 - It sounds like you have a transmission problem. This has nothing to do with whatever the check engine light is indicating, which might be other problems. But as far as the transmission is concerned, I would recommend that you take the car to a reputable transmission place, not to a general car mechanic.
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.
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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.
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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.
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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
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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.
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Julie Varner - Your state inspection will fail if the check engine light is on. It's best to find out ahead of time what the code is, so you know what's wrong. You can have a mechanic read it for you, if you trust your mechanic. Or you can get you own diagnostic tool to read it, like the one I mentioned, which will cost less than the service to read it for you. And then you can be sure you know truthfully what the issue is. Good luck with 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
- 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!
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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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Wade - Thanks for your feedback. The Diagnostic Code Scanner will help you know what's in your car's computer so you can do it yourself in the future.
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
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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.
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brutuses - This happens. There are honest dealers and there are crooks. Both exist. Nothing new.
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.
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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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Ernie, if all the lights are off, then it will pass the inspection assuming the computer is not showing any "not-ready" conditions left over from a previous code that may have cleared itself. If you want to know for sure before bringing in your car, get one of the code scanners and check the "ready status."
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.
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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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Matt, Your mechanic may be talking about another item or they are taking advantage of you. For the small price of a new gas cap and the small cost of buying your own code scanner, it pays to try that first. After using a new gads cap, clear your computer codes and watch to see if the light comes back on.
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?
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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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Jim, This is the reason for reading instructions first. I can't tell you if you did any damage. That all depends on the type of car you have and the way the code reader you purchased works. The vgate is not one I listed in my article.
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
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Joe, Since you had trouble with the engine it is unlikely that it's just a sensor.
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?
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Liam, You mentioned that you had code P0420 show up, so tell your mechanic you want the sensor replaced that deals with that code. But just the sensor, not the cat converter.
Happy Holidays to you too.
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!
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Liam, code P0420 implies that the catalytic converter is bad. But since the light went off by itself then it is most likely just a bad sensor. You passed the inspection because the computer cleared itself just in time for the inspection. It does that if the sensor stops sending the bad diagnostic code long enough. It may continue to fluctuate on and off. I wouldn't run out and spend $1000. Save your money. If you want you can get a new sensor installed. But don't fall for a $1000 job.
Happy holidays to you too! Thanks. Best of luck with your Honda, that's a great car!
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!
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Mike, Sure sounds like a sensor problem. It may have been stuck and released itself when no power was applied during the battery change. You saved $1000.
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?
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Jenny, they must be talking about the ignition coil which creates the high voltage required for a spark.
In older cars there was one coil with a distributor to each cylinder's spark plug. Today's cars use one coil per cylinder, either mounted on top of the spark plugs or in a common block with high tension wires to each spark plug.
Some cars share one coil between two cylinders, using a technique known as the "wasted spark" system. But I won't go into detail as that is a different discussion.
Glad you got your problem resolved. Thanks for reporting back.
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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