One problem related to catalytic convertor has been reported for the 1999 Audi A4. The most recently reported issues are listed below.
At 51,000 miles I had the catalytic converter replaced on my Audi under warranty due to a broken pipe flange. I now have 102,000 on the Audi and the same part is broken. Audi USA will not replace or repair this part. I feel it must be a defect in the workmanship or materials since a catalytic converter should not be replaced until it is worn out and not before then. This is part of the emissions system and the epa should also know about this.