BMW X4 Exhaust System Troubles, Help Needed Urgently

Hey there! I had a similar issue with my 2012 BMW X3 last year. I have some experience working on cars, mostly basic maintenance stuff. The symptoms you described are nearly identical to what I experienced. My X3 was running rough, had poor acceleration, and the check engine light was constantly on. I initially thought it was the catalytic converter since I had that classic exhaust smell too. Took it to my trusted workshop, and after running diagnostics, they found that the lambda sensor (oxygen sensor) was shot. They replaced it, and the car's been running like new since then. Total bill came to 375 Euro including parts and labor, not cheap, but definitely better than replacing the whole catalytic converter! Could you share some more details about your X4? Like the mileage and whether you've had any previous exhaust system repairs? Also, did you get any specific error codes from the CEL? This would help confirm if we're dealing with the same issue.

Thanks for sharing your experience! My X4 has 128009 KM on the clock and just had its service last month. I got the error codes checked yesterday, it's showing P0420 and P0430. The exhaust system is all original, haven't had any repairs done to it before. Starting to think it might be more serious than just sensors, but your experience with the lambda sensor is giving me hope it might be a simpler fix. Did your mechanic run any specific tests before concluding it was just the sensor?

Hey again! Thanks for those details about your X4. Yeah, those error codes are definitely familiar, I had P0420 on my X3 too. Actually, when my mechanic dug deeper into my case, they did run a few specific tests. They used a digital multimeter to check the voltage readings from both oxygen sensors (upstream and downstream) and performed a back-pressure test on the catalytic converter. Initially, they suspected it might be the cat, but the back-pressure test showed it was still functioning okay. The voltage readings from my lambda sensors were way off though. The downstream sensor was giving really inconsistent readings, which explained the rough running and poor performance. The interesting thing is, bad sensors can often mimic the symptoms of a failed cat converter. Since your symptoms are so similar to what I experienced, and you've got similar codes, I'd really recommend getting those sensor readings checked before jumping to any conclusions about needing a new catalytic converter. These BMWs can be tricky with their exhaust systems, but sometimes the fix is simpler than we expect. Let me know what your mechanic finds, I'm really curious if it turns out to be the same issue! The whole sensor replacement really sorted out my problems, and I'm hoping you get the same result.

Thanks for the detailed info! You've definitely got me feeling more optimistic about this whole situation. Just called my mechanic and scheduled an appointment for tomorrow morning. Asked them specifically to check the sensor readings like you mentioned before making any conclusions about the catalytic converter. Really hope it turns out to be just the sensors, would save me a ton of money! I'll update the thread once I get the diagnosis. Your experience with the X3 seems so similar to what I'm dealing with, especially with that P0420 code. Thanks again for taking the time to share all those details about the tests they ran on yours. Really helps to know what to expect when I take it in tomorrow. Fingers crossed it's a straightforward fix like yours was!