P0420 Audi - 2001 A4 Quattro 1.8T.

Question

My first thought is O2 Sensor. Is that correct? Any other suggestions??

Answer 1

P0420 Bank 1 catalytic convertor not effective

This is also a VAG code 16804, with Audi you clear the code and generate a readiness code, if this code appears again then you replace the catalytic convertor.