BMW M4 O2 Sensor Issues: Symptoms & Solutions

Hey there! I have some experience working on BMWs. Actually, I had a somewhat similar issue with my 2017 BMW M3 last year. The symptoms were pretty much identical to what you're describing with your M4. In my case, it turned out to be a faulty connection in the oxygen sensor wiring harness. My mechanic found that the plug connection was corroded and not making proper contact. Got it fixed for about 95 Euro, and it solved all the issues, the check engine light went off, fuel consumption went back to normal, and emissions were fine again. Before I can give you more specific advice, could you share: How many miles/kilometers are on your M4? Did these symptoms appear suddenly or gradually? Have you noticed any rough idling or hesitation during acceleration?

Thanks for the detailed reply! My M4 currently has 135371 KM and had its last service about 3 months ago. The issues started gradually over the past few weeks, and yes, I've definitely noticed some rough idling, especially when the engine is cold. The acceleration feels a bit hesitant too, particularly between 2000-3000 RPM. That's really helpful to know about the wiring harness, I'll make sure to have my mechanic check that specifically. Did you notice any specific driving conditions that made your symptoms worse?

Oh those symptoms you're describing are exactly what I experienced with my M3! The rough idle and hesitation between 2000-3000 RPM were particularly annoying. In my case, the symptoms definitely got worse during cold weather and early morning starts. Also, when I was driving in stop-and-go traffic, the rough idling was much more noticeable. The hesitation was really frustrating, especially when I needed that quick acceleration for merging or passing. Just to add to my previous reply, after getting the wiring harness fixed, my mechanic actually recommended checking the connections every 60,000 KM or so. He said these issues are pretty common in our engines, especially as they age. The corrosion usually builds up gradually, which explains why the symptoms tend to appear slowly. Given your mileage (mine was around 128,000 KM when this happened), I'd bet you're dealing with the same issue. The fact that your problems started gradually and got worse over time matches my experience perfectly. Make sure your mechanic thoroughly checks all the connections, not just the obvious ones. Sometimes these electrical gremlins can be tricky to track down! Let me know what your mechanic finds, I'm really curious if it turns out to be the same problem!

Thanks so much for sharing all those details about your experience! It's incredibly helpful to know that you had such similar symptoms, especially the cold weather and stop-and-go traffic issues, that matches exactly what I've been experiencing. I actually haven't paid attention to the time of day, but now that you mention it, the rough idling is definitely worse during my morning drives. I've already called my mechanic and scheduled an appointment for tomorrow morning. I specifically mentioned the wiring harness corrosion issue you encountered, and he said it's definitely worth checking first given the similarities. That's a great tip about checking the connections every 60,000 KM too, I'll definitely add that to my maintenance schedule going forward. Really appreciate you taking the time to share your experience. I'll update the thread once I get the diagnosis and repair done. Fingers crossed it's just the wiring harness and nothing more serious!