BMW M3 NOx Catalyst Issues, Power Loss & Exhaust Leaks

Hey there! I have some experience working on cars, and I actually faced a similar issue with my 2008 BMW M5 last year. Really sounds like what I went through! The symptoms you're describing match exactly what happened to me, engine light, power issues, and those pesky emissions problems. My local workshop found a leak in the exhaust system near the catalyst, which was messing with the readings and causing the power loss. Got it fixed for around 265 Euro and my M5 has been running smooth since then. The repair took about half a day. Before I can give more specific advice, could you share: Any specific error codes you're getting? Is there any unusual noise from the exhaust? Have you noticed any fuel consumption changes? These details would help me understand if it's the same problem I dealt with. The M3 and M5 share quite a few components in their exhaust systems.

Thanks for sharing your experience! Had my last service just 2 months ago at 184244 KM. I'm getting the P0420 error code, and yeah, there's this annoying rattling sound from the exhaust, especially when cold. Fuel consumption has definitely gotten worse, I'm burning through about 15% more fuel than usual. My mechanic mentioned something about the catalyst being the likely culprit, but wanted to get some real-world feedback from other BMW owners before committing to the repair. The price you mentioned seems reasonable, gives me a good benchmark for what I might be looking at.

Hey again! Thanks for those details, that P0420 code and the symptoms you're describing are exactly like what I dealt with on my M5. The rattling sound especially brings back memories! When I had the issue, my mechanic first thought it might just be a sensor problem, but after a deeper inspection, it was definitely the catalyst. The extra fuel consumption you're seeing (15% is pretty close to what I experienced too) is a classic sign, the engine has to work harder because the catalyst isn't doing its job properly. Quick update on my repair, it's been about 8 months since I got it fixed, and everything's still running perfectly. Fuel consumption went back to normal, that annoying rattling disappeared completely, and my last emissions test was a breeze. The repair cost I mentioned earlier turned out to be money well spent. One thing I'd suggest though, make sure they check the oxygen sensors while they're at it. My mechanic found that one of mine was getting a bit iffy, and replacing it at the same time saved me another potential headache down the road. Let me know how it goes with your M3. These cars can be a bit demanding with maintenance, but they're totally worth it when they're running right!

Thanks so much for all the detailed info! Just booked an appointment with my mechanic for next week. I'll definitely have them check those oxygen sensors too, really good tip! It's such a relief to hear someone had the exact same issue and got it sorted without any major drama. Your experience really helps put my mind at ease about the repair costs and timeline. Will keep you posted on how it goes with my M3. Really appreciate you taking the time to share your experience, these BMWs are amazing machines when they're running right, and I can't wait to get mine back to its best performance!