Read engine fault code after it has been turned off?

So here’s a strange one. One can use an ODB reader to turn off a check engine light and, for a few dozen miles, cancel an engine fault code. Is that fault really cancelled, or am I just turning off the light? That is, once that light is turned off, can one still pull the original error code? I’m out to get a used car from a random person, and it would be nice to know if some fault has been so cancelled. I guess if there is a fault, the light will pop back on after a few dozen miles, but I’d rather not wait that long. Maybe there is some super ODB code reader that can do that?

on your scanner it should tell you if emissions testing is ready. if it says its not then the codes were deleted or the battery was disconnected.


I don’t know your budget and it is none of my business . But if you are concerned look at the web sites of Carmax and some of the other big used vehicle retail concerns. Thay do have some kind of warranty and some have limited time to return the vehicle if you don’t like it . Of course this is still the worst time to be car shopping.


Erasing the fault codes will extinguish the check engine light, that is the only was to switch off the light with a code reader and I am not aware of a “cancel” feature.

Erasing the fault codes also clears the emission monitors. If all of the emission monitors are shown as “passed” or “ready” then there is no reason to suspect tampering.

You could take $15,000 to Carmax and buy a vehicle with an extended warranty (which some say are worthless). I get more satisfaction buying a vehicle in the $2000 to $5000 range.

Thank you. Not sure my question was answered, though. If the check engine light is off, maybe someone just used an ODB reader to shut it off, and there is still a problem. Is there some way to check those codes AFTER the ODB kills the check engine light? Or does the ODB really completely clear any code, such that if there is a problem, the problem is undetectable until, many miles later, it pops back on?

If the error is cleared and the problem still exists, it will set the CEL and store another code. How quickly depends on how intermittant it is. There is no super code reader that can read recently cleared codes. The hint that they were recently cleared is the message that the emissions test is not yet ready.

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if you know the codes were cleared why would you even want to buy a car that they were trying to cover a problem up. if it was a easy fix they probably would of fixed it.

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There is a misunderstanding. The code reader can’t directly turn off the check engine light. The code reader can be used to erase fault codes, the PCM will then switch off the check engine light.

To determine if the fault memory has been cleared, check the emission monitors;

This is the standard operating strategy for all OBDII systems so that a vehicle operator can’t clear the system faults a few minutes before an emissions test.

You are an experienced mechanic where the rest of us might think a vehicle is decent only to find out we made a huge mistake. In my area even a 5000.00 dollar vehicle might not even get you home.

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Of course, someone selling a car can turn off the check engine light by disconnecting the battery or by using a code reader to clear codes. This is, of course, one of the most common scams pulled by people selling a used car privately, and is actually one of the easiest-to-detect scams that you need to watch out for.

The way to combat this scam is by bringing an inexpensive code reader with you. When you show up to test-drive the car, start the engine and plug in your code reader. Navigate to “I/M Readiness” and see if all the monitors show as “ready” or “not ready”. If the codes were recently cleared, several monitors will show as “not ready”. In most states, model years 1996 through 2000 can pass emissions testing with up to two monitors showing “not ready” and model years 2001 and newer can pass emissions with up to one monitor showing “not ready”.

Be aware that there are other more dangerous scams out there, which could result in your buying a car that you will never be able to legally register and drive. One of the biggest scams is people selling cars with “jumped” title, meaning that the person selling the car is not the person named on the title. They may offer all kinds of excuses why this is the case, for example, they might claim it’s their boyfriend’s/girlfriend’s car, etc. If you buy such a vehicle, and make any mistake filling out the title, or the previous owner made any mistake, you won’t be able to contact the owner of record, and thus will have to go through the bonded title process (if available in your state), or otherwise junk the car.

Another big scam which I encountered was people selling cars with electronically recorded liens against the title. Some of these vehicles had government debts owed, such as fines and restitution or child support, others had online title loans. Fortunately, here in Arizona, the state offers a lien inquiry website, where you type in the VIN, pay a nominal fee, and can then see if there are any liens or government debts attached to the car. I had more than one person try to claim that they had clear title, and of course they had a physical title, but an online title loan was taken out against the car. When I brought this to the sellers’ attention, they played dumb and promised to “correct the error” but then quit returning my calls.

Another scam is that people selling a car which leaks oil/coolant/transmission fluid, or is in the beginning stages of head gasket failure will not allow you to come to their home, or otherwise have the opportunity to start the engine from cold. They will offer all manner of excuses why they don’t feel comfortable letting you come over, and instead want to meet in a public place–which means the engine is already warmed up, and you have no opportunity to see evidence of leaks in the usual parking spot. Every vehicle I have owned which had a head gasket go bad, the initial problem was that it was very difficult to start the engine from cold, and it ran very poorly when cold, but then ran better once warmed up.

I remember several years ago, I went to a man’s house to look at a Dodge Caravan from the mid-2000s. It was being sold for way more than Blue Book value, firm price, based on the fact that it was supposedly in mint condition and ran flawlessly. I brought my code reader with me, as well as my wife and son for the test drive.

I remember that the van was difficult to start, and had barely any fuel in it according to the gauge. I wanted to take a long test drive, but the low fuel light came on after a few miles. I observed that while stopped at a light, the engine ran very roughly, and after driving for 11 miles, the check engine light came on. When I got back to the seller’s house, he told me it was just a sensor, and would cost less than $75 to fix–then proceeded to connect his code reader and cleared the codes. Needless to say, I did not believe a word he told me, and told him very bluntly that if it was such a cheap and easy fix, then logically he would have already fixed it–rather than going to such lengths to cover up the problem.

You must live in a super-high cost of living area. Even now, in this overpriced used car market, I can find a plethora of used cars for $3k or less, which I’d be willing to drive on a trip of 500 miles or more.

Or maybe because there’s an ongoing global pandemic, I don’t want strangers that I know nothing about anywhere near my family and home…or I just in general don’t want strangers anywhere near my family and home…or my house is in an alley and is difficult to get to if you don’t know where you’re going so it’s just far more convenient for all involved to meet in a public place (a lot of our local police stations for instance have parking lot areas for just such transactions). There are more reasons to choose to meet in a public place than there are not to. If someone was really worried about it, I’d park the car someplace overnight and drive over in my other car, but they’d better be damn serious about the car and I would need to be selling it for $5k+ I’m not doing all of that for a $2,500 or less beater.

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OK, thank you, everyone. The answer is that YES, someone can use an ODB reader to turn turn off the codes, and thereby turn off the check engine light AND, there is no way I can check the car for existing faults that would have set that code without waiting for the car to set them on again. That is, there is no way for one (with ANY equipment) to poll the car - “Hey, are you SURE there are no faults?”

I think the lesson here is that, in buying a used car from an uncertain seller, with no faults displayed, it would be smart to get, say, a 100mile warranty on it. If the car had a fault, the code would probably get reset within that mileage.

Now, if someone had the equipment to poll the car, I’d be happy to spend a few bucks to ask them to do it before shoveling money at a seller.

you can buy your own scanner for as little as $25 and do it yourself.

some code readers (and based on the data pids available,) can even tell you how many miles have been run since the DTC memory was cleared. If it is 25 miles, it is more sketchy than if it is 2500 miles.

I’d put no chance of being able to enforce that with a random individual.


What price reange are you looking at . Random individual ? A private seller is not going to give even a 1 mile warranty and many used dealers have a sold as is policy .


That’s not quite accurate. If an OBDII scan shows there are no diagnostic codes AND all of the readiness monitors are complete, that means the computer knows of no faults present.

If one or more of the readiness monitors are incomplete, that means the computer isn’t sure whether or not faults are present, and is waiting to find out by acquiring more data as the car is driven.


Well, let’s make up a situation here.

Let’s say I buy a used car. No check engine light. Looks great! I hand over the money,and get the car. Twenty miles down the road, the check engine light fires. Oops. P0420. Let’s see, that means $2K in catalytic converter replacement. Um, no wonder they were selling it. They just turned the check engine light off.

So my options are …? What should I have done to avoid that situation? Ah, just buy a car from a dealer with a warranty?

The fact that cheap ODB readers can turn off identified faults is a buyers nightmare.

that’s why you check the readiness monitors. If the cat is bad, and they just cleared the codes for the test drive to hide that, the readiness monitors will not be set.

a good mechanic can (and should) check all of this in a pre-purchase inspection.

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