Big thank you to you guys who could help or suggest. 2001 facelift Audi S3 8L with 210bhp AMK engine. The ECU was hard chipped by the previous owner and cannot be reverted (I don't know anything more than this on this matter). Failed MOT due to high emission. I have read previous threads on this subject advising a few actions to begin with; I have cleaned the 2 cats and the pre-cat lambda sensors, to bring cat temp. up before testing etc... Readings improved slightly (to about 3.5 of CO) but still well over. here are the readings: CO at idle 4.090 %Vol CO at high idle 4.540 %Vol Lambda 0.871 HC 0.00 ppm Vol From the threads I have read and some research on the web, I suspect it's too rich in combustion, could anyone with knowledge in this area help to determine what's causing this high level of CO emission please? Is it the old school chipping of the ECU or different sensor faults etc... One mechanic told me it might be cause by damaged injectors, which worries me a lot. Much appreciated.