I have a 1999 A6 Avant quattro. I have a rattling sound coming in the area of the catalytic converter and I had a check engine light come on and got the following computer code from my friends diagnostic readed: code is P0432 - Main catalyst below threshold (bank2) that I reset, but I still have the rattling sound during engine rev in gear or out of gear, which is getting worse. Does this mean the catalytic converter is bad? How do you determine which converter is bad if their are 2 of them? Can the front or back or the bad converter be lowered to take out the lose object inside making the rattling noise and if so, will it still pass the emissions?
sounds like it, this happend to me in my jetta, the catalyist in the converter broke and rattled around. You can replace it or do what i did and take it off and empty out/chip out the rest, but its illegal.
I know this is a long time after you have posted your article. You prob had it replaced by now. That was def the honeycomb insides of the cat crumbling into little pieces. I had the same on my A4 2.4. I removed both cats and replaced it with straight pieces of stainless pipes. Big difference in power, and not too much of a noise increase. But then again its prob illegal where you are...here in South Africa its ok, as long as dont make your neighbour's dog jump 6ft heigh when you blip the gas pedal.
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Rattling sound from catalytic converter - 
