I got a clutch job done on my 2003 Audi A4- car ran beautiful. Then a few months down the road, with no warning, my Audi started sounding a "tractor" and of course the check engine light came on. At first, I thought it might be bad gas, however, I am careful where I purchase gas, and always use premium. Anyway, I got a diagnostic check- Codes, P0420 (catalyst system efficiency, air leak near sensor) My now mechanic, checked the catalylic converter and noticed "three bolts missing on it- near the upstream O2 sensor! We could not see how three bolts just "fell" off or loosened. Which led me to believe, when I had the previous work done, they "forgot" to put the bolts back on- This is where the 02 sensor air was coming from. My mechanic replaced the bolts and changed the ECT-Engine Coolant Temperature (which had went out). The car ran just like when I first got it. However it was still reading the Code P0420- so we replaced the front (upstream) O2 sensor. I hope this helps anyone who maybe experiencing the same thing- Oh yeah, when you are ordering or buying parts for your Audi, make sure you get OE (Original Equipment), try to stay away from "Universal Parts" or fits. They may be cheaper, but you get what you pay for. And one more thing, find a primary mechanic for your Audi- Ever heard the term "too many cooks in the kitchen" is the same way with your Audi- "Don't have to many mechanics under your hood"-