Just bought a 02 A6 Quattro 3.0 w/ 91k miles. Great shape, but check engine light is on again w/ the message, "Cat Converter below threshold". Replaced both sensors, also the guy I bought it from replaced the cat converter (after market, not Audi). Any idea why the check engine light keeps coming on? She runs great, mileage is great...hmmmm

 

I had one of my cats replaced at the Audi dealer yesterday. It was replaced for free under the federal 8 yr/ 80,000 mile warranty. I don't know if it is your after market cats, but my check engine light has not come back since the repair.

Check this discussion out:
https://www.audiforums.com/m_595766/tm.htm