BMW 3-Series AUC Sensor Failure Causing Odor & Fan Issues

Had the exact same problem with my 2005 BMW 5 Series diesel last summer. The AUC sensor died completely while stuck in traffic. Got pretty nasty fumes coming through the vents, and the blower stopped working too. Took it to my regular mechanic, turned out to be a corroded connection between the sensor and the main wiring harness. Moisture had gotten in there over time. They had to replace both the sensor and part of the wiring. Not a super expensive fix, but it needed immediate attention because those fumes can be dangerous. The repair took about 3 hours. They cleaned up the whole area and applied some protective coating to prevent future corrosion. Haven't had any issues since then. If you're getting the same symptoms, don't wait too long to fix it. It's not one of those problems you can just live with, especially with a diesel engine. Get a proper inspection of the entire ventilation control system while they're at it.

Thanks for sharing your experience! That's really helpful to know it was the corroded connection in your case. Mine's showing pretty much identical symptoms. Really glad to hear yours was sorted out properly, peace of mind knowing it's fixable! Would you mind telling me roughly how much you ended up paying for the whole job? Also, has everything stayed working fine since the repair, or have you noticed any other ventilation quirks?

Hey there! Just wanted to follow up, took your advice and got it checked out. You were spot on about not waiting too long! Mechanic found the same issue, it was just a faulty cable connection to the AUC sensor. Only cost me 95 € to fix, which was way less than I was worried about. The best part is, everything's working perfectly now. No more nasty fumes coming through the vents when I'm stuck in traffic, and the blower's back to normal. Been about three weeks since the repair, and there haven't been any weird ventilation issues at all. Really glad I didn't put it off, you were right about not ignoring these kinds of problems, especially in these older BMWs. The mechanic also gave everything a good clean and check while they were in there, just like you mentioned. Thanks again for sharing your experience, helped me know exactly what to expect!

Hey, appreciate you letting me know how it turned out! Actually, I just got mine fixed last week too. Went with a new mechanic this time, what a difference! They were super thorough and found it was exactly the same problem. The funny thing is, I was freaking out thinking it would cost a fortune, but it ended up being really reasonable. Just needed to clean up the connections and replace a small part. The whole thing took about two hours, and the best part? No more weird smells or ventilation problems! The mechanic walked me through everything they did, which was refreshing after my last experience. They even showed me what to watch out for in the future. Car's running like a dream now, feels great to have fresh air coming through again when I'm stuck in traffic! Thanks again for helping me avoid making this into a bigger problem than it needed to be. Sometimes these older BMWs just need a bit of TLC!