So, I have an 2012 A6 Quattro 3.0 and my check engine light has come on 2x for the P0420 Cat below Threshold (in July and August). I dropped the car off at the dealer (still under warranty for another 5K miles), and they immediately said it was likely the 02 sensors which are not covered.

They are running diagnostics. I have since researched and found several references, comments, etc. that state unless there is an accompanying 02 error code, that the it is very likely that the Cat needs to be replaced.

My concern is that the dealer charges me to replace the 02 sensors and 6k miles from now when my warranty expires my P0420 code returns and I am out of luck.

How do you confirm that it really is the 02 sensors and not the CAT or vice versa. Trying to make sure I am not taken here.

Thanks,

Newbie Audi Owner