I have been having this problem with my V8 2001 x5. from SES lights going on to oxygen sensors being replaced. Now I finally have taken it to the dealer and their telling me now that I have to replace the left cat. at an estimated $2000 cost. I went to a muffler shop who quoted me way far less for a universal cat. my question is has anyone replaced their oem cat with a universal cat and have they had any problem? let me know please I want to fix the problem so I can enjoy my truck without all of the noise coming from the cat.