BMW 6 Diesel: NOx Sensor Issues & Failed Emissions Test

Hey there! I have some experience with these issues as I went through something very similar with my 2014 BMW 5 Series diesel (and I've also owned a 3 Series). What you're describing sounds exactly like what happened to my car last year. The NOx sensor symptoms were identical, check engine light, failed emissions, the whole deal. Turned out it wasn't actually the sensor itself, but a damaged wire connection to the NOx sensor that was causing the false readings. My mechanic found a corroded plug that was causing intermittent connection issues. The fix was pretty straightforward, they replaced the wiring harness and connector. Cost me 95 Euro and solved the problem completely. Before I can give you more specific advice, could you share: Which engine variant you have? What's the exact error code showing? Have you noticed any other symptoms, like reduced power?

Thanks for sharing your experience! Mine's at 157058 KM and the last service was just done a few weeks back. You've mentioned something really interesting about the wiring, I've been having occasional power dips, especially when accelerating, which could definitely point to an electrical issue rather than the sensor itself. I was convinced I'd need to replace the entire NOx sensor unit, but maybe I should have my mechanic check the wiring and connections first. That would be a much cheaper fix! Just to clarify, did you notice any weird engine behavior before finding the corroded plug? Mine sometimes feels a bit jerky when cold.

Hey again! Absolutely, your symptoms are ringing even more bells now. Yes, I definitely had those power dips too, and the jerky behavior when cold was exactly what I experienced! It's almost uncanny how similar this is to what my 5 Series was doing. I remember being really worried it was going to be an expensive sensor replacement, but honestly, those power dips were the biggest clue that it was electrical. The jerkiness when cold was particularly bad in my case, sometimes it would even stutter a bit before smoothing out after a few minutes of driving. After seeing your mileage, mine was actually pretty close, around 165000 KM when this happened, I'm even more convinced it's probably the same issue. The wiring tends to deteriorate around this mileage, and since you mentioned having a recent service, it's possible that some movement during maintenance might have aggravated an already weak connection. Here's what I'd suggest based on my experience: have your mechanic specifically check the wiring harness near the sensor first. In my case, you could actually see some green corrosion on the connector when they pulled it apart. Don't let them jump straight to replacing the sensor, that would've cost me about 800 Euro if I hadn't caught the real issue! Quick question, have you noticed if the power dips happen more when it's damp or rainy? That was another tell-tale sign in my case.

Thanks so much for all this detailed info! You've basically described my situation to a T, and yes, now that you mention it, the power dips do seem worse in damp weather! That's another sign pointing to the electrical connection issue. I'm feeling much more optimistic now knowing it might just be a wiring problem rather than a full sensor replacement. I've already called my mechanic and specifically asked them to inspect the wiring harness and connections before jumping to any conclusions about the sensor itself. They've booked me in for tomorrow morning. Really hoping to see that same green corrosion you mentioned, never thought I'd be excited about finding corrosion, but it would mean a much cheaper fix! Thanks again for taking the time to share your experience. It's saved me from potentially spending hundreds unnecessarily on a new sensor when the real culprit might be just a damaged wire. I'll definitely update once I get the diagnosis!