I owned a 2002 Audi A4 3.0 up until a couple of years ago and that kept throwing up a catt efficiency code. There was some talk about the perameters being set too tight right from the factory. It always passed it's emissions part of the MOT easily.
The constant MIL light did bug the hell out of me though so I had to sort it. I tried new pre and post catt O2 sensors but to no avail. Catta clean in the fuel, also no good. I didn't want to spend the £1500 on a new down pipe and catts so waited for a second hand one to come up for sale. I took the car and the replacement catt to a local custom exhaust shop, paid em £100 to cut out the old catt and weld in the replacement. All done in situe on the car. No more problems.
I have actually had to do the same on the S8 but that was because of a rattling catt.
A problem you may come across is actually finding a second hand catt. Most breakers sell em on for recycling quite quickly after removal.
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Audi S8 D2 2002 88k mls
Ebony pearl effect black. Grey interior.
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