BMW Z4 Exhaust Issues & Failed Emissions, Help Needed

Hey there! I've got some experience working on BMWs (some experience with maintenance and repairs). Had a similar issue with my 2011 BMW M3, and my cousin owns a 335i. The symptoms you're describing sound exactly like what I dealt with. Mine turned out to be a faulty lambda sensor. Had rough idling, poor performance, and that annoying check engine light. Took it to my regular workshop, and they diagnosed it pretty quickly with their diagnostic tool. The repair, including parts and labor, set me back about 375 Euro, but it fixed all those issues you mentioned, the rough running, exhaust smell, and emissions problems. Could you share more details about your Z4? Like, did these symptoms appear gradually or suddenly? Any other warning lights? This would help narrow things down, as BMWs can be tricky with these symptoms. Quick fix at the shop and the car's been running great since. Just make sure you get it checked soon, as driving with a bad lambda sensor can cause more expensive problems down the road.

Thanks for sharing your experience! My Z4 just hit 144868 KM and had its last service three weeks ago. The symptoms started gradually over the past month. No other warning lights besides the check engine. Kind of relieved to hear it might just be the lambda sensor and not something more serious. Will definitely get it checked asap, already booked it in for next week. Hoping it'll be a straightforward fix like yours was. Really appreciate the heads up about potential complications if left unchecked.

Hey again! Glad you got it booked in. Yeah, after dealing with my M3's issues, I'm pretty confident it's the lambda sensor, especially since your symptoms appeared gradually, that's exactly how mine started. At around 140k km, these sensors often start showing their age. Just want to share a follow-up from my experience, after I got mine fixed, I noticed the car felt way more responsive, like night and day difference. The fuel economy improved quite a bit too, which was a nice bonus. Since you mentioned having service done recently, definitely let the workshop know about that, sometimes these issues can pop up right after routine maintenance if something got knocked loose. One thing I learned from my experience, if they do confirm it's the lambda sensor, might be worth replacing both upstream and downstream sensors at the same time. I initially only replaced the faulty one, but ended up having to go back six months later for the other one. Could've saved myself a second trip to the workshop! Keep us posted on what they find. Always good to share these experiences with other BMW owners. And hey, at least it's probably not something super expensive like the catalytic converter!

Thanks so much for the detailed insights! Just got back from the workshop with my Z4. You were spot on, it was indeed the lambda sensor! They found both upstream and downstream sensors were failing, so I took your advice and had them replace both. Really glad I did. Total bill came to 425 Euro for both sensors plus labor, which feels fair considering the alternative scenarios I was worried about. The difference in how the car runs now is incredible. Engine's super smooth again, that nasty exhaust smell is gone, and the throttle response is back to normal. Fuel consumption has definitely improved too, already noticed I'm getting more miles per tank. Really grateful for your help and especially that tip about replacing both sensors at once. Saved me from a potential second workshop visit! Still keeping an eye on things, but it feels great to have the car running properly again. Thanks again for sharing your experience, made me feel a lot more confident about getting it sorted!