BMW 5 Series NOx Issues: Check Engine & Failed Emissions

Hey there! I'm a hobby mechanic with some experience, and I actually faced a similar issue with my 2004 BMW 3 series (also had the 7 series at the time). The symptoms you're describing sound really familiar. Mine turned out to be a faulty cable connection to the NOx sensor. Had it checked at my regular workshop, and they found the wiring was corroded, causing false readings. The fix was pretty straightforward, they replaced the wiring and connections, cost me around 95 Euro. But I can't say for sure if your issue is identical without more info. Could you share: Current mileage? Any specific error codes? Notice any changes in fuel consumption? Any unusual exhaust smell? This would help determine if you're dealing with the same problem I had or something else.

Thanks for the detailed response! My current mileage is 148281 KM and I just had the regular service done last month. The fuel consumption seems about the same, but I am getting a weird sulfur-like smell from the exhaust. Haven't pulled the error codes yet, planning to do that tomorrow with my OBD scanner. The corrosion issue you mentioned makes sense since the car's been through quite a few winters. Your experience gives me hope it might be something relatively simple and not too expensive to fix. I'll check those connections you mentioned before heading to the shop.

Hey again! Thanks for those details, that sulfur smell you mentioned immediately rings a bell! When I had my 3 series, that same smell showed up right before I discovered the wiring issue. Given your mileage and the winter exposure, I'm even more convinced it could be similar to what I dealt with. Quick tip from my experience, when you're checking those connections tomorrow with your OBD scanner, take a really close look at the wiring harness near the NOx sensor. In my case, the corrosion wasn't super obvious at first glance. I actually had to wiggle the connections a bit to notice the issue. The sulfur smell in my case cleared up completely after fixing the wiring. One thing I learned the hard way, if you do spot corroded wires, don't just clean them and hope for the best. I tried that first and ended up having to replace them anyway two weeks later. Complete replacement is the way to go. Let me know what codes you pull tomorrow. If they're anything like what I got (I think mine was P0420, but it's been a while), I might be able to give you more specific pointers from what I learned dealing with my car. Just be careful with those winter-worn connections, they can be pretty fragile when you're checking them!

Thanks so much for all these valuable insights! This information about checking the wiring harness and the specific tip about not just cleaning corroded wires is super helpful. I really appreciate you sharing your experience in such detail. The fact that you also had that sulfur smell makes me feel more confident about what I might be dealing with. I'll definitely take your advice and be extra careful when checking those connections tomorrow. Will make sure to do a thorough inspection of the wiring harness like you suggested, rather than just pulling the codes. And good call on the complete replacement, you probably saved me from making the same mistake with just cleaning the wires! I'll let you know what I find out with the OBD scan. Even if it turns out to be something different, you've given me a solid starting point. Really helpful to have someone who's been through something similar share their experience. Thanks again for taking the time to help out!