M3 NOx Sensor Failure Leading to Failed Emissions Test

I had a similar issue with my 2018 BMW M4 last year. The symptoms were nearly identical, check engine light, NOx-related codes, and emissions problems. In my case, it turned out to be more than just a sensor issue. The wiring harness connecting to the NOx sensor was damaged, causing intermittent connection problems. The insulation had worn through on a section where it was rubbing against a bracket. Found this during the second visit to the garage after the first shop just wanted to replace the sensor straight away. The repair involved replacing the wiring harness assembly and the NOx sensor since it had been damaged by the electrical issues. Not a cheap fix, but definitely better than replacing the whole catalytic converter, which is what I initially feared. The symptoms cleared up completely after the repair, and I haven't had any emissions-related issues since. Based on your description, I'd suggest having someone specifically check the wiring and connections before jumping to sensor replacement. It's worth being thorough with the diagnosis since these systems are pretty complex.

Thanks for sharing your experience with the wiring harness and sensor replacement. It's really helpful to know it could be more than just the sensor itself. Remember what the total cost was for your repair? Also curious if you've had any other issues pop up since getting it fixed? My symptoms do sound pretty similar to what you dealt with. Think I'll get that wiring checked out before jumping into any parts replacement. Trying to avoid throwing money at the wrong solution.

Hey there! Just wanted to follow up on your questions about the repair costs. Actually, in my case it turned out to be much simpler than my initial wiring harness issue I mentioned before. When I took it back to a different mechanic, they found it was just a loose connection at the NOx sensor plug. They fixed the cable connection and properly secured it, total bill came to 95 €, which was a huge relief! Since getting that sorted, the car's been running perfectly, no more check engine lights or emission issues at all. It's been about six months now, and I haven't had a single problem pop up. Nothing like what I experienced with the sensor before. Much better outcome than when I thought I needed the whole wiring harness replaced! Sometimes it really pays to get that second opinion, right? Really glad I didn't rush into more expensive repairs. Hope this helps with your decision making!

Thanks so much, what a relief to hear it might be something as simple as a loose connection! Just got back from a new mechanic actually, and you won't believe it, turned out to be almost exactly what you described. Found a loose connector and some minor corrosion around the sensor plug. Cleaned up the connection points, secured everything properly, and cleared the codes. Bill came to just over 100 bucks for the diagnostic and fix. Such a massive difference from what the first shop quoted me for a full sensor replacement! Been driving it for a few days now, and the check engine light hasn't come back on. Exhaust seems normal again too. Really glad I asked for advice here and got that second opinion instead of going with the first repair quote. Could've spent way more than necessary. Thanks again for pointing me in the right direction!