Car has 63K miles. Engine light came on, had it checked at Auto Zone, pulled a code P0431. There is no noticeable difference in preformance and no bad smell. I find it hard to believe the cat is bad. I'm open to suggestions. Especially ones that dont involve thousands of dollars. I'm the second owner of the car, so I'm not sure if warranty work will happen for me. THANKS!

 

If your cat is good then there's a trick - add a spacer in the O2 sensor after the cat. There are 2 pre-cats codes p0421 is one side, p0431 is the opposite side. Sometimes people get both codes at same time.

www.bigdaddiesgarage.com

The after cat sensor is taking data to compare the data with pre-cat sensor to tell the cat's efficiency. If you don't have rattling, rotten egg smell, big drop in performance, then the chance of bad cat is low and the Audi ecu specs are too sensitive to pop a code.

Also give Audi dealer a call to see if your cat is still under Fed Emission Warranty. If it still is then Audi will replace the cat FOC under warranty.

 

I'm pretty sure cats are covered up to 80k miles regardless of mileage. Check with a dealer. My daughter's 2003 A4 3.0 MT6 went in on the coil recall in Dec 2009 and they found a bad cat at 79,124 miles and they were both replaced "free".