Like a lot of the A6's, mine was throwing the code for cat troubles. I was not excited about buying the OEM cats at $1500 since that sounded more like treating the symtom and not the desease.
So how's this for strange?
The wife backs the A6 into a steel post and crunches (among other things) the muffler on the left side. The shop replaces that (and about $3000 worth of other parts) and the fault code has been out ever since, about 4 months.
I don't understand why, but I'll take it.

 

That is an interesting problem that I would also like to here the answer to. I am also having issues with the cats on my 00 A6 4.2. I was just going to bite the bullet, but know I am curious.