MOT fail this morning on emissions

[Greegsy]

Just had MOT this morning and failed on emissions as shown. CO readings sky high and lambda readings low. Car runs spot on with no error codes/dash lights. Only had car for around 6mths and at last MOT had quite a bit of work to pass MOT including new lambda sensors as it actually failed on the same, exhaust carbon monoxide at last MOT. I fitted a milltek stainless system about 1 month ago - could that cause this issue? Looking at receipts it was lambda sensor 1 and 3 that were replaced for last MOT. What does this mean? Any help/suggestions please?

[Shoppinit]

Did he try the probe in both sides? Were they the same? Does your exhaust have cats in it?

[Greegsy]

Thanks for quick reply. Unsure if he tried both sides and yes cats are still fitted to car. Was only a 15 minute drive to garage so maybe not fully heated up? Oil temp was sitting at around 80ish on gauge

[srichards]

What fuel is normally used in it?

[Greegsy]

I just stuck £40 of Tesco momentum in the other day

[srichards]

Is the SAI system still on it or has it been removed and coded out properly? I had issues with mine popping up the EML and being a bit stinky until the map was tweaked properly. Been fine for a while. The old SAIs fill with water then die.

[Greegsy]

Thanks for input, ok think we are getting warmer….car does sound like a jet when starting up and it’s the SAI making all the noise for a minute or so when started from cold. No fault codes showing though but could this be the culprit causing dodgy emissions?

[srichards]

Thanks for input, ok think we are getting warmer….car does sound like a jet when starting up and it’s the SAI making all the noise for a minute or so when started from cold. No fault codes showing though but could this be the culprit causing dodgy emissions?

Highly possible. Needs to go to an RS6 place really or at least a VAG specialist.

[Mɐʇʇ]

Not convinced. SAI runs for just a couple of minutes on startup to help warm the cats a bit quicker. But once the car is warmed up is no longer operational and won’t impact emissions.

I have an SAI coded out map somewhere here that I made years ago, I removed my SAI about 8 years ago.

Probably need to check the primary and secondary lambda readings with VCDS to work out if it’s running a bit rich, or if the cats are not working.

[Shoppinit]

To narrow down the cause, I would want to know if both banks were reading the same. If they are then the cause is likely in the admission. If not then in the exhaust.

I'd also want to know if the car's lambda sensors were reading the same as the gas analyser. If there's a significant different that could point to a dodgy sensor or a pre-sensor exhaust leak can cause wonky lambda readings. The SAI could be a source of this if a combiner valve has failed, letting air into the exhaust manifold.

If the car has been modified, then who knows what warnings have been coded out.

Also check the trim to see if one side or both is running rich.

[Mɐʇʇ]

Yeah, good point L to R as well. And I was thinking looking at the reading of both Lambdas each side would help correlate. But main point of my post was so say I'm less convinced it's SAI related since, whether coded on or off, since it only runs for a couple of minutes. Could certainly just be an iffy map though couldn't it.

[Shoppinit]

Yeah, I don't think it's SAI either. Unless it's a combiner valve, and they do fail. But that would mean readings off on just one bank, unless very unlucky. Similarly, one bank being off could implicate an injector, spark plug, coilpack, etc.

I suppose the new cats could be been incorrectly installed, allowing a leak on one or both sides.

I guess with the tools you've got available, you can check the pre and post lambda sensors readings with the Vagcom, on both banks. That will give us a bit more info than we've already got. That and the trim, and MAF readings. Belt and braces, innit.

Can you still buy exhaust gas anlysers? You used to be able to buy them analysers from Halfords in the days when you could adjust your own mixture.

[Mɐʇʇ]

Can you still buy exhaust gas anlysers? You used to be able to buy them analysers from Halfords in the days when you could adjust your own mixture.

I used to have a gunson one that came from halfords. I never found it much cop albeit was good enough for "gross" adjustments. These day's anything I am making I just put an innovate LC2 into it : https://www.innovatemotorsports.com/lc- ... -8-ft.html

They do have a standalone unit, but it's a bit pricey : https://www.innovatemotorsports.com/lm- ... e-kit.html

This is not bad though, I have one of these too : https://www.innovatemotorsports.com/mtx ... -8-ft.html
And then maybe : https://www.innovatemotorsports.com/o2- ... clamp.html (not got one of these since I always just weld or otherwise fit a bung into the exhaust)


But - don't you reckon the rear lambdas on the car are reliable enough? And can be replaced in situ if there's any doubt?

[Greegsy]

Thanks for all your input guys. Car is booked in on 11th December at independent specialist to hopefully resolve these issues. I’ll keep you posted….

[philS2]

Mine is rich on bank 2 and just gets through every year.
It has always read about 5g/s compared with bank 1 which is about 3.5g/s.
Maf swap and it stays on bank 2.
Maf wire extension and it swaps to bank 1 (so obviously wiring)
I've renewed maf wires with no change to resistance or voltage on either, but comparing both mafs wiring, there was some discrepancy (can't remember what)
So I traced it to the ECU.
Had to open the security fixings.
But once in there I cleaned all contacts and (probably, it was a while ago) checked resistances from Ecu plug to each each maf wire.
No joy, so that was that.
Sorry if you read this and I was f@ck all help.