On my Mom's 1999 M3 the catalytic convertor went bad 5 months ago, and was replaced. She opted for the non BMW branded replacement because it was $500 less. 2 months later the catalytic convertor went bad again. She bought the BMW branded one this time. Now this one has gone bad and it has only been three months! The shop that does the work specializes in BMW's and said that it is impossible to have two defective catalytic convertors so there must be some other problem, possibly with the engine. The only code that the computer shows is for the catalytic convertor. Anyone have any experience with a problem like this or have any idea what me going wrong?