Hello
Audi TT 2001 180 hp
A while back i had an engine code referring to catalyst below efficiency. The dealer recommended a new CAT.
I bought and fitted a cat.
Now I have 0171 code (too lean).
I've changed the rear O2 sensor.
Cleaned the MAF sensor.
New fuel filter.
Injector cleaner.
Repaired a vacuum leak going to the brake servo, plus new dipstick with a good seal.

I can re-set the MIL light etc, but then it misfires on all cylinders, until the ECU works out it's too lean, then it puts the MIL on, throws 0171 code......but runs really well!

Any suggestions for my next move??
Thanks,
Simon