Yesterday, the wife's '02 A4 3.0 Quattro Manual (89K) got a CEL with the dreaded fault code P0420 (cat converter efficiency below threshold). My independent Audi mechanic cleared the code, but cautioned that I would likely get it again. He said that the tolerances in the control module are programmed too tightly, and the ECM throws the code at the slightest variance in O2 readings behind the cat. Says he has seen cats replaced under warranty in response to this fault code, only to have the code reappear within a year or so. Audi, so far, hasn't developed a new flash program with looser tolerances.

I love the car, but hate the problems and repair expense. I'll trade the thing before I start down the slippery slope of cat-converter replacements! That is, unless someone out there has a solution.