Oxygen sensor faulty without a fault on the OBC

[Dino D]

Not sure if this VW specific or applies to other makes but will ask here to test the waters.

The car in question is a VW Eos 1,4TSI 08 model. When in for a service the dealer says that the oxygen sensor is faulty and needs replacing. The car is 2 months out of its 2 year warranty so not covered. Has done 23k miles.

The car drives smoothly and there is no warning light coming up on the dash. In my experience in other cars (including VW's) when there has been a problem with the Lambda sensor (is that the same as the oxygen sensor?) the engine management light would come on and/or the car would run rough.
Is the service department telling porkies or is it possible to have a faulty oxygen sensor and not notice any running or issues or have a light come on?
If that is the case how the heck are you supposed to know the sensor is knackered? I assume it has an important role in the function of the engine?

I don't generally distrust main dealer service departments or am calling them liars but they did try and get my mother in law to agree to changing the pads on the car at a cost of 280 euros + labour so I think are trying their luck a little hence I wonder about this sensor story.

[DaveL86]

Lots of fault codes can be logged on the OBC ( ECU ) without causing a light on the dash or display, from my experience of owning VAG cars they only show dash lights or errors when its pretty serious and will show something similar to "STOP/SERVICE/WORKSHOP/EMISSIONS", a completely failed oxygen sensor might trigger an "EMISSION WORKSHOP" error on the dash, but one reading slightly out of spec might just log the error on the OBC, the only way to know if codes are logged is with a hand held code reader or ideally VCDS (formally VAG-COM).

Part of a full service will usually involve a diagnostic of the OBC, its pointless not to since they have to hook the computer upto the car to clear the service indicator and finding codes can make them money, the only real way to tell if they are truthful is finding someone independent or you trust to read the codes from the OBC/ECU.

[Chiswickboy]

Ask the dealer for a copy of the emissions test and post the results here. I'm sure someone will know what the figures mean.

alternatively, for a second opinion you can go to another garage and ask them to do an emissions test and compare results.

[B'stard Child]

Chiswickboy said:

Ask the dealer for a copy of the emissions test and post the results here. I'm sure someone will know what the figures mean.

alternatively, for a second opinion you can go to another garage and ask them to do an emissions test and compare results.

I'm not sure emission test would be meaningfull when looking downstream of the cat - might be wrong but hey ho

[Scuffers]

B'stard Child said:

Chiswickboy said:

Ask the dealer for a copy of the emissions test and post the results here. I'm sure someone will know what the figures mean.

alternatively, for a second opinion you can go to another garage and ask them to do an emissions test and compare results.

I'm not sure emission test would be meaningfull when looking downstream of the cat - might be wrong but hey ho

if the (primary) lambda is working, then a tail pipe test should show a lambda reading of 0.95-1.05.

if the secondary lambda has failed, it will not show up like this, it's only there to verify CAT function.

as said, ideally you need to see the fault code.

[Superhoop]

How are they saying it's faulty?

Are they saying there is a fault code stored, or just that they believe it's faulty, and if the latter, based on what?

The MIL has to illuminate on a car that is EOBDII compliant (The Eos will be) if there is an emissions related fault - If the Lambda sensor was faulty, it would affect emissions, so should illuminate the lamp

[Dino D]

Hi All,

Thanks for the in-depth advice. The car has passed an emissions test as part of the service so it cannot be that serious of an issue now but maybe will develop into one (you have to have one annually here and keep the certificate in the car).

I will speak with dealer to find exactly what is wrong and ask for some sort of contribution as it just 2 months out warranty. I think it will be a good idea to go to an independent too for a second opinion.

Is it true that poor fuel can damage these sensors?

[Chiswickboy]

B'stard Child said:

Chiswickboy said:

Ask the dealer for a copy of the emissions test and post the results here. I'm sure someone will know what the figures mean.

alternatively, for a second opinion you can go to another garage and ask them to do an emissions test and compare results.

I'm not sure emission test would be meaningfull when looking downstream of the cat - might be wrong but hey ho

Surely the whole point of the sensor is to feed back data to adjust the fueling and therefore control emissions.

The cat takes out some of the products of combustion but it will not do it properly if what is going in is not within the Lambda range ie not the correct fuel to air ratio. Overfueling can bugger up the cat quicker than a rabid dog.