BMW 5 Series Exhaust and Performance Issues: Help Needed

I had a similar issue with my 2017 BMW 3 series (gasoline). Having some experience working on cars, I first thought it was the catalytic converter, but my trusted mechanic diagnosed a faulty lambda sensor. Those symptoms you're describing, rough idle, poor acceleration, and emissions problems, match exactly what I experienced. The repair was pretty straightforward, paid 375 Euro for the new sensor and labor. Car's been running smoothly since then, and the check engine light disappeared immediately after the fix. Can you share your engine size and current mileage? This would help determine if we're dealing with the same issue, as these sensors typically fail at certain mileage intervals. Also, have you scanned for error codes? That would really help pinpoint the exact problem.

Thanks for sharing your experience! My 5 series has 17825 KM on it since my latest service. I actually got it scanned at a parts store yesterday and got the P0420 code, which seems to align with what you mentioned about the lambda sensor. The timing of these issues matches too, started right after a cold morning start. Did you notice any specific sounds or smells before getting yours fixed? The repair cost seems reasonable compared to what I was worried it might be. Really hoping it's just the sensor and not the entire catalytic converter!

Hey again! Yeah, those symptoms you're describing sound exactly like what I dealt with. When my 3 series had the lambda sensor issue, I definitely noticed a strong sulfur-like smell, especially during cold starts. There was also this subtle rattling sound at idle that went away once the sensor was replaced. The P0420 code is pretty telling, that's the exact same code I got. Since your mileage is relatively low at 17825 KM, I'd bet money it's the lambda sensor and not the cat. These sensors can fail prematurely sometimes, especially if you've been using lower-quality fuel or if there's been any water damage. One thing I didn't mention in my first reply, before getting it fixed, I noticed my fuel consumption went up by about 15%. Did you notice anything similar? After the sensor replacement, my fuel economy went right back to normal. Just make sure your mechanic checks both upstream and downstream sensors, as sometimes they can misdiagnose which one's failing. In my case, it was the downstream sensor that was shot. The repair should take them less than an hour if they know what they're doing. Hope this helps!

Thanks so much for all the detailed info! You're spot on about the fuel consumption, I've been filling up way more frequently lately but hadn't connected the dots. Just checked my fuel tracking app, and I'm burning about 20% more fuel than usual. That's really reassuring about the mileage too, I was worried the whole catalytic converter needed replacement, which would've been a much bigger hit to my wallet. I'll definitely mention both sensors to my mechanic when I take it in tomorrow. Really appreciate the heads up about having them check both upstream and downstream, wouldn't have known to ask about that. The sulfur smell I'm getting is identical to what you described, and now that you mention it, I can hear that subtle rattle at idle too. This has been super helpful, feeling much more confident about the repair now. I'll let you know how it goes after the shop looks at it. Thanks again for taking the time to share your experience!