My daughter's 2002 A4 (60K miles) CEL has been on for a few weeks. We read the code (P0050 - O2 sensor heater circuit bank 2 sensor 1) and replaced the O2 sensor. (What a job! But that's another story.)
I disconnected the battery to reset the memory, but the code returned quickly. I then had the code read a second time, with the same result, P0050. This time I had the OBD memory reset by the code reader. The CEL was out for a short time, but next time the engine was started cold, the code again returned.
I'm stumped. Can anyone suggest what else I should look for?