BMW NOx Sensor Issues: Diagnosis & Fix Guide

Hi there! I have some experience working on cars and had a similar issue with my 2002 BMW 5 series. The NOx sensor problem is pretty common in these models, including the 3 series. I took my car to my regular workshop when I noticed the check engine light and failed the emissions test too. Turned out it was just a faulty cable connection to the NOx sensor. The mechanic checked the wiring, found a corroded plug, and replaced the connection. Total bill was 95 Euro, not too bad considering I was worried it might be the catalyst itself! Before I can give more specific advice, could you share: Engine size? Mileage? Any other warning lights? Have you noticed any change in fuel consumption? This will help me understand if you're dealing with the same issue I had or something different.

Thanks for the input! I have the 2.5L gasoline engine with 187228 KM on the clock. Actually had my last service 2 months ago but this issue just started recently. In my case, I'm getting weird readings on the OBD scanner that suggest the NOx sensor voltage is fluctuating. Haven't noticed much difference in fuel consumption but the exhaust smell seems stronger lately. Did anyone else experience the smell issue before fixing their sensor? The car still runs ok otherwise, just concerned about potential damage if I keep driving it like this.

Hey again! Yeah, I remember dealing with that exact same situation on my 5 series. The exhaust smell you mentioned definitely rings a bell, I had that too before getting it fixed. With those OBD readings you're getting, it sounds pretty similar to what I experienced. My voltage readings were all over the place too. But here's something I didn't mention in my first reply, after the mechanic fixed the wiring, we actually found that the sensor itself was partially damaged from moisture getting in through that corroded connection. Given your mileage (mine was around 180k when this happened) and the strong exhaust smell, I'd bet you're dealing with both a connection issue and a worn sensor. The smell got worse for me when the sensor wasn't reading properly, causing the system to run rich. Don't want to scare you, but I kept driving mine for about two weeks with those symptoms before getting it fixed, and it started affecting my fuel trim values. Would recommend getting it checked soon, the sensor itself isn't super expensive, but the longer you wait, the more likely you'll face additional issues. By the way, did your mechanic check the exhaust manifold during your last service? Sometimes a small leak there can mess with the NOx readings too. Just something to consider.

Thanks so much for the detailed response! That's really helpful, especially about the moisture damage you found. I'll definitely get it checked out ASAP, don't want to risk any additional problems. You've actually made me feel better knowing you dealt with such a similar issue. I just called my mechanic and booked it in for tomorrow morning. I asked about the exhaust manifold check too, good thinking on that! He said they'll do a complete diagnostic including checking for any leaks. The symptoms you described match mine exactly, so I'm hoping it's just the sensor and wiring like in your case. Really appreciate you taking the time to share your experience. I'll make sure to ask them to check both the wiring connections and the sensor itself. Better to fix it now than deal with bigger problems later. Thanks again for the help!

Hi have a BMW e87 gasoline 2008 where I have error code Nox sensor has changed it but error code comes back after a few days the car runs as it should otherwise it is sensitive that it must be some specific Nox sensor mvh Pelle

(Translated from Swedish)

Check if the replacement NOx sensor is compatible with your specific BMW model, as using an incorrect sensor can cause recurring error codes. Also, inspect the wiring and connectors for any signs of damage or corrosion that might be affecting the sensor's performance. If the issue persists, it might be worth checking the control module or software updates related to the NOx system. Let me know how it goes or if you have any further questions!