i recently purchased an 03 A6 3.0L 106k w/ no CEL.

after filling tank i was driving home and CEL came on. went to auto zone to get the code pulled and it came back P0421, after looking through the forums i found that this is a common problem.

however, probable cause was rich air/ fuel ratio. is there a way i can correct this or am i looking at a new cat

just bought the car, 1st AUDI = completely clueless

any advice is good advice, thanks

 

That code could be caused by a number of things, such as a vaccum leak, torn throttle body boot, bad O2 sensor or even a faulty catalytic converter. I would diagnose it by starting with the more simple and easiest things first. Check for any signs of a vac leak, or tear in your throttle body boot before shelling out hundreds of dollars to buy O2 sensors or new cats that you may not need.

 

do some searching here there have been some posts regarding this type of problem I beleive it had something to do with fuel getting into the evaporitive canister then pushing back to the engine through the vacum lines going to the canister. The canister is located underneath the car in the spare tire well area there is a plastic cover over it. If I remember right the easy way to check it was by weight if it was ok it would be rather light because it is just full of fine charcoal but if it was full of fuel it would be rather heavy. being that your light appeared right after fueling up this could be your issue.