apparently there is a sensor on the oem downpipe /cat that is nt on the milltek , has any one fitted one them selves to see this ?
The only sensor in the downpipe/CAT is the O2 sensor, which swaps straight over, or did on my 08 model year S3. You just cant swap it back as it welds itself into the SS exhaust.
Ref the original question, I have heard this(but not sure where from), but Ive always mapped first, before exhausts anyway. Give Charlie at Milltek at call, he is very helpful and certainly answer your question.