My Audi dealer told me I need both Cats replaced, but when I pulled error codes myself I got only P0421 which is bank 1 only. I think they're trying to screw me by replacing both. Anyone dealt with this error or cat replacements?

Also, I've heard this problem could be due to a post cat O2 sensor as well which would be a heck of a lot cheaper than a new cat. Is there a way to test that theory?

Thx,
2002 A4 3.0 Q

 

You only need to replace the passenger's side one (bank 1) but in the case of the 3.0L you probably need to replace both cats. The reason for this is cat converters go bad from misfires, the 3.0L has terrible ignition coils that misfire accross the board when they get older so chances are the other cat isn't far behind.

The O2 sensor theory is bogus, it just records the efficiency of the cat converter. Plus our engine management systems can tell the difference between a bad O2 sensor and cat converter.

 

I just got this same code... But my car just got back from the body shop, and they were supposed to replace both Cats... Any other problems this could be?

I was rear ended and the exhaust got all beat up!