Failed MOT

[cookie7354]

Hello all, went to MOT my car today (Audi cab 2.6 on a 94 plate) and it failed on one thing the fast idle on the emissions? It was ok on normal idle. It had a new cat this time last year so im open to ideas, any help appreciated

Many thanks

[S2AVANT]

Have you got Vag Com?
Lambda sensor / Maf ?

[cookie7354]

No will they tell me exactly whats not working, the car goes back into Borobridge garage on Friday so hopefully they will check that. Is the VAG the reader which tells you all the problem codes?

[Dudey Head]

I had trouble with 94 coupe 2.6 last year with emmissions. It was OK at certain engine speeds but not others. It's got a custom built 3" diameter exhaust system including cat (Jetex custom build parts). I initially thought that the cat would be to blame with it not being the original spec & having giving lower back pressure which I thought maybe was confusing the lambda sensors. I had it back into the garage that put the exhaust together & they found a little leak. Replaced the lambda sensors for good measure & then it was fine.

[cookie7354]

another week and the lambda probes are fine so it must be the cat. Thing is it had a new cat fitted this time last year and has only covered 4000 miles! I will keep you all updated incase you ever come across this

[rally5]

As an mot examiner I may be of some help, If you take a perfectly healthy car and remove the Cat the reading you would get on the CO is 0.5% with the MOT limit of 0.3%, However if the reading is in excess of 1.0% the you will have an engine management fault, I.E. Lambda sensor, Temp sensor or MaF sensor etc
If it failed on a Lambda reading only then its most likely caused by an exhaust leak or rarely a too weak a mixture.
Hope the helps, but please post your readings.

[cookie7354]

Rallye5 many thanks for that it was the Cat in the end, the old one had only lasted 4000 miles and under a year so im going to try to get me cash back.