Hello,
Bought my wife a 2002 A4 Avant with 5 speed auto and 3.0 V6. The car has 96,000 miles. Just had a ton of service done on it including one of the 02 sensors on Bank one. Can't remember which one. So after all the work was done I got a CEL and the code turned out to be PO421, bank one cat on warm up. The shop that did the work cleared the code and said take it fora long ride and see if the code goes away. Maybe they suspected a carbon build up. So I did what they said and the code stays away for a few weeks but now it is back. Does this mean I need a cat? They look expensive to get to, and they don't look easy to sneak an aftermarket in there. The car is otherwise stock. Any ideas? Thanks, Joel

 

if you do need a cat, that is one expensive *** job on the 3.0

 

Yeah cats frequently throw fault codes for the 3.0L engine, as for the 1.8,2.8,2.7, and 4.2 engine there is a TSB to re-flash the ECM because the O2 sensors are too sensitive and throws a fault code at the first sight of danger. This has been corrected on all these engines but no the 3.0L engine yet, don't ask me cause i don't know why. You might wanny try taking it in to an emmisions place and have them analyze the emmisions coming out of the tail pipe. If the PPM is in acceptable limits for that state then i would try installing some non-foulers or spacers on the rear O2 sensors so it won't throw a fault code. Do a search for test pipe installs, they usually involve non fouler solutions.
As for the cats, if you want to replace them the parts are over $1,000 a cat, and it take about 5 hours of labor time to put them in bringing you to a grand total of around $3,500.
Which is not ideal.

 

Thanks for the informative reply. I'll research the non-foulers. I could tell by the Bentley description of replacing a cat that it would be an expensive fix. Merry x-mas to me!