Check Engine - O2 Sensor? Or fuel?
 

I just purchased a 2000 A6 Quattro 2.8 with the check engine light on.

I was told by the previous owner it was an O2 sensor, but I also found out he was running regular fuel instead of the premium fuel as per the owner's manual.

I just had the codes cleared at the local service centre and I will see if they return.

My question is: could the codes have been caused by running the incorrect type of fuel?

Codes cleared were: PO421 bank 1; PO431 bank 2; P1176; P1423; and P1411.

Thanks.