BMW M4 NOx Sensor Woes: Fix Advice Needed

I had a similar problem with my 2014 BMW M3 last month. The NOx sensor issue showed up just like yours, alongside the check engine light and emissions concerns. At the garage, they found that the wiring harness connecting to the NOx sensor was badly corroded and had some loose connections. The sensor itself was actually fine, but the faulty connection was causing erratic readings. They had to replace the entire wiring harness and clean the connector ports. It wasn't a critical issue for immediate driving safety, but definitely needed fixing to prevent potential long-term damage to the emissions system. The repair took about 4 hours to complete. My advice would be to get it checked soon, these issues typically don't resolve themselves and can lead to bigger problems if left unchecked. Make sure whoever works on it has experience with BMW emission systems. They'll need proper diagnostic tools to verify it's fixed correctly after the repair.

Hey there, thanks for sharing your experience! That wiring harness issue sounds exactly like what I might be dealing with. Really helpful to know it wasn't the sensor itself in your case. Could you remember roughly how much you ended up paying for the whole repair? Also, has everything been running smoothly since they fixed it, or have you noticed any other issues pop up?

Hey again! Glad to give you an update on how things turned out with my M3. After getting it fixed, I can tell you it was definitely worth it and way cheaper than I initially feared. Only paid 95 € to fix the faulty cable connection to the NOx sensor, turned out to be just like I suspected from my previous post about the wiring issues. The repair has been absolutely solid since then. The check engine light disappeared right away and hasn't come back. Emissions are running perfectly now, and the car feels much smoother overall. No other issues have popped up related to this at all. Looking back, I should've mentioned in my first reply that these wiring problems are pretty common on these models. The good news is that once it's properly fixed, it usually stays fixed. The key was finding a mechanic who knew exactly what to look for instead of just throwing parts at the problem. If your symptoms match what I had, you might be looking at a similar simple fix rather than needing a whole new sensor. Much better for the wallet that way!

Thanks so much for all the details! Just got back from the mechanic today and wanted to close the loop here. You were spot on about the wiring, took my car to a new shop that specializes in German cars. Turns out I had almost the exact same problem. The mechanic found corroded wiring near the NOx sensor. Total bill came to about 120 € for the wiring repair and diagnostic work. Way better than the 500+ € I was quoted for a full sensor replacement at the other place! Car's running like a dream now. Check engine light's gone, and the exhaust smell has completely disappeared. Really glad I asked here first and got your advice, saved me from unnecessarily replacing the entire sensor. Definitely keeping this mechanic's number for future issues. Thanks again for pointing me in the right direction. Case closed on this one!