I have a 01 A6 2.7t...and it keeps throwing code P0431 every couple days after i erase it...I know that its either a cat going bad or a bad 02 sensor...where at and which side is it located because i was planning on fixing it myself. Thanks

 

Basically this means that the oxygen sensor downstream of the catalytic converter on bank 2 is detecting that the converter is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

A code P0431 may mean that one or more of the following has happened:

* The catalytic converter is no longer functioning properly
* An oxygen sensor is not reading (functioning) properly
* There is an exhaust leak

Possible Solutions

Inspect for exhaust leaks. Next step is to measure the voltage at the oxygen sensor on Bank 2. In fact, it would be a good idea to test each oxygen O2 sensor while you're at it.

 

P0431 on 2.7 can also be triggered by the low threshold on the ECU. The dealer can reflash the ECU.