Hi, just a general question here. If the ECU is reset to remove any warning lights, is it still possible for a reader to pick up any current issues or does the car have to be ran for the required cycles to the warning lights come on again before a reader can pick it up?

The reason why I am asking is because, it seem like quite common practice by both private sellers and dealerships to reset the ECU and then sell the car "faultless" only for you to get the check engine light after a few days.

Cheers

Unless the fault condition is resolved, the code will return immediately after being cleared (either by code clear or ECU reset).

Ok, so if I understand you correctly, the code will return immediately but it just wont show up as a check engine warning on the dash board until you have driven for a certain amount?

If an error is detected, it will trigger the EML on the dash and the error code will be stored in the ECU. For the light to not come back immediately, the seller would have to either fix the fault or put a bodge in place.

In my case, I bought the car and there were no warning lights showing. After 2 days, the check engine light came on. I returned to the dealer who gave me some vague answer about car running a bit lean or something like that (obvioulsly they picked up that I knew nothing about cars and the error code was erased. Fast forward about 3 more days and the check engine light comes on again and I return yet again and they say that they cant find out whats wrong but nothing major anyway and erase the code again. I get tired of it and stop pushing and then just before my 3 month warranty engine and gear box warranty run out, I go up once again and then they all of a sudden know that it error code P0420 and that the cat needs replacement but its not covered under the warranty. Quoting me about €800 to get it fixed (only a few month left until NCT). So I say thanks but no thanks and get in touch with other mechanic who make a cheaper fix by whelding on a cat after the regular one getting my car throught the NCT. However, the car still obviously know that the original cat is faulty, thus keeping the check engine light on. However, everytime I get something fixed and error codes are erased/ECU reset it takes about 3-4 days before the check engine light come on again.

So my question is, if I would have taken the car to a 3rd party mechanic before buying it, would they have been able to see the P0420 error code even though the check engine light was not on?

Found this on another forum, if correct it could explain why the cat related issue does not generate an immediate check engine light after ECU reset.

"When you reset the computer (by clearing any codes), these readiness blocks are reset as well and your car will not have had time to check them out; typically it takes a couple days of driving to fully check out these functions (the cat for example needs to get really hot for the car to check its functionality)."

Ah yes, the exception to the rule
The cat itself won't be generating the error, but one of the lambda sensors (pre cat or post cat) will be.
Whether it's the sensor itself which is faulty, or something causing the sensor to pick up non-standard readings, I can't say.

Edit: Actually I doubt it's the sensor itself as it would throw the errors immediately if it was. If the first garage said it was running rich, then that's probably what's causing the spurious lambda readings. Some investigation will need to be carried out into what's making it run rich.

Thank you for your response I believe it is the sensor picking up non-standard readings as the car did not initially pass the the emissions test. Ideally, I would get a new proper cat which would fix the check engine light, but not sure if its worth spending that much on the car when its now running fine. Another option would be to check if I could extend and fit the sensors on the secondary cat (dont even know if its possible).

Again, thanks for taking the time to reply

The CAT error checking does take a few cycles of the engine running. So it would be indicative of the cat itself.
You can prevent the error from being registered by using one of these to extend the second lambda sensor.

It's not ideal, but it's only 7gbp.
Not sure how MOT views it...probably not ok.

Sometimes the cat is not 100% dead, and can be cleaned revived by longer hard runs, some people add some lacquer thinner or things like Redex fuel system cleaner... to the fuel.

But if you are only driving it short distances it won't be ok.
I killed a CAT on my Bravo by doing about 12-15 km /day to work in the morning and then back home in the evening (for about 1 year). it was definitely not enough to get it to operating temperatures and it eventually got clogged and died=> P0420 -Catalitic efficiency
Temporary solution was the O2 extender.

SikWitIt123 said:

So my question is, if I would have taken the car to a 3rd party mechanic before buying it, would they have been able to see the P0420 error code even though the check engine light was not on?

Click to expand...

No, they could not have detected the error.
On the Fiats you can't see stuff like pending tests..etc like on some american / Japanese cars.

Thanks Aurick,

I guess I was just a bit unlucky that it was a fault which was possible to hide. Anyways you live and learn

Great to have a forum like this with helpful people to give pointers, much appreciated for all the help everyone.

aurick86 said:

You can prevent the error from being registered by using one of these

Click to expand...

Its quite interesting that they seem to be completely unavailable in Ireland. Not like it is much hassle to order but should be possible to find them somewhere here

Get one from aliexpress, should be there in ~3 weeks for 3 euro.
Only thing it needs to be M18x1.5 and have the small hole through it ~2mm

Maybe I will just order a few, if someone else will be looking for them down the line haha