clearing fault codes

Shilton · Jan 27th, 2006, 19:16

Just a quick one does the engine/emission management light go out after the fault is fixed, or after the fault is fixed and the codes cleared.

Only I sometimes get the light on when I am at a standstill in traffic with the engine running but when I stop and clear the fault code (2-1-2 a faulty oxygen sensor) from the ecu the light goes out until the next time it happens (which may be days or weeks) always with the same fault code the obvious question being are the emissions when on tickover outside of the parameters set by the ecu and the light can be ignored or is it a faulty oxygen sensor?

dgbalfour · Jan 27th, 2006, 22:27

I believe the code only has to be cleared to put the light out. If the ECU thinks the faults are fixed then the light won't come back on again.

That said, I think there are some codes that disappear if the fault is fixed. For example, as my last battery was dying I'd often get the Lambda light come on for a few seconds as the voltage was too low, but this went out as soon as a the volts came up - the code was then erased.

Maybe your idle speed is wrong causing an incorrect emissions reading?

David

Peter D · Jan 28th, 2006, 16:05

Do you have a scope or a digi meter. You could connect the digi to the Lamda output up on the left of the brake servo, there is a 2 pin connector for the power feed and a single spade connector for the output. With the engine warm and at tick over you should observe voltages between 0.1 and 0.9 of a volt. they cycle about every 1 to 3 seconds. if you are getting a fixed voltage then the sesor is not working. The other problem can be a lazy sensor taking 10's of seconds to change voltage.This is much easier to see with a scope. Replacement senors available for £40. Regards Peter

Clifford Pope · Jan 29th, 2006, 11:51

In general, the light stays on until the code(s) are cleared. Then next time you start the computer does its usual search for all the right signals, so if the problem has been fixed, no more light.
But if the poor or missing signal only shows up later, perhaps until particular circumstances occur, then the light comes on again. It sounds as if yours may be in that category.
A poor oxygen sensor signal can be shown by no more just accumulated carbon on the sensor. If you can get it unscrewed from the cat you can very cautiously vacuum it and try again. That can also be done from the front end of the cat if you have occasion to remove the down pipe or replace the part of the system attached to the cat.
Incidentally, if you do need to unscrew the oxygen sensor, I found a good method was if you get the whole cat section out on the bench, and clamp the sensor hexagon very firmly in a good stout vice. Then just lean on the pipe to unscrew the cat from the sensor. I once attempted to undo mine in situ and found even a good box spanner was just rounding the flats on the sensor.
Having got it free once you can put on a tiny dab of copperease for next time.

Shilton · Jan 30th, 2006, 19:39

Thanks for that info the Haynes manual is sometimes a bit sketchy about these sorts of things, unfortunatly I have to run the car up onto railway sleepers and work by the roadside but I think I will wait for the weather to get a bit warmer first.

Jan 27th, 2006, 19:16	#1
Shilton Junior Member Last Online: Jun 5th, 2013 19:16 Join Date: Dec 2003 Location: london	clearing fault codes Just a quick one does the engine/emission management light go out after the fault is fixed, or after the fault is fixed and the codes cleared. Only I sometimes get the light on when I am at a standstill in traffic with the engine running but when I stop and clear the fault code (2-1-2 a faulty oxygen sensor) from the ecu the light goes out until the next time it happens (which may be days or weeks) always with the same fault code the obvious question being are the emissions when on tickover outside of the parameters set by the ecu and the light can be ignored or is it a faulty oxygen sensor?

Jan 27th, 2006, 22:27	#2
dgbalfour VOC Member Last Online: Yesterday 13:59 Join Date: Jan 2006 Location: Bedford	Engine fault codes I believe the code only has to be cleared to put the light out. If the ECU thinks the faults are fixed then the light won't come back on again. That said, I think there are some codes that disappear if the fault is fixed. For example, as my last battery was dying I'd often get the Lambda light come on for a few seconds as the voltage was too low, but this went out as soon as a the volts came up - the code was then erased. Maybe your idle speed is wrong causing an incorrect emissions reading? David

Jan 28th, 2006, 16:05	#3
Peter D Peter D Last Online: Mar 2nd, 2015 21:03 Join Date: Dec 2002 Location: Livingston	Do you have a scope or a digi meter. You could connect the digi to the Lamda output up on the left of the brake servo, there is a 2 pin connector for the power feed and a single spade connector for the output. With the engine warm and at tick over you should observe voltages between 0.1 and 0.9 of a volt. they cycle about every 1 to 3 seconds. if you are getting a fixed voltage then the sesor is not working. The other problem can be a lazy sensor taking 10's of seconds to change voltage.This is much easier to see with a scope. Replacement senors available for £40. Regards Peter

Jan 29th, 2006, 11:51	#4
Clifford Pope Not an expert but ... Last Online: Apr 26th, 2024 12:45 Join Date: Dec 2001 Location: Boncath	In general, the light stays on until the code(s) are cleared. Then next time you start the computer does its usual search for all the right signals, so if the problem has been fixed, no more light. But if the poor or missing signal only shows up later, perhaps until particular circumstances occur, then the light comes on again. It sounds as if yours may be in that category. A poor oxygen sensor signal can be shown by no more just accumulated carbon on the sensor. If you can get it unscrewed from the cat you can very cautiously vacuum it and try again. That can also be done from the front end of the cat if you have occasion to remove the down pipe or replace the part of the system attached to the cat. Incidentally, if you do need to unscrew the oxygen sensor, I found a good method was if you get the whole cat section out on the bench, and clamp the sensor hexagon very firmly in a good stout vice. Then just lean on the pipe to unscrew the cat from the sensor. I once attempted to undo mine in situ and found even a good box spanner was just rounding the flats on the sensor. Having got it free once you can put on a tiny dab of copperease for next time.

Jan 30th, 2006, 19:39	#5
Shilton Junior Member Last Online: Jun 5th, 2013 19:16 Join Date: Dec 2003 Location: london	Thanks for that info the Haynes manual is sometimes a bit sketchy about these sorts of things, unfortunatly I have to run the car up onto railway sleepers and work by the roadside but I think I will wait for the weather to get a bit warmer first.

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