BMW 5 Hybrid AUC Sensor Failure Causing HVAC Problems

Hi there! I have some experience working on cars and had a very similar issue with my 2015 BMW 7 Series Hybrid (also owned a 3 Series before). The symptoms you're describing sound exactly like what I experienced. My blower was acting up and I got those nasty smells too. Turned out the AUC sensor cable connection was loose and partially damaged. The workshop diagnosed it quickly and replaced the wiring harness for the sensor. Total cost was 95 Euro and it fixed the problem completely. Before we dig deeper into potential solutions, could you share: Has the check engine light come on? Which error code specifically are you getting? Does the issue happen more when it's humid outside? This will help me better understand if you're dealing with the exact same problem I had.

Thanks for the detailed response! My latest service was just two months ago at 118685 KM, and nope, no check engine light. Getting error code 804A21 on my scanner. The smell and blower issues definitely get worse during humid weather. Really hoping it's just a wiring issue like yours and not the entire sensor unit that needs replacing. Just worried about those repair costs, BMW parts aren't exactly cheap! Have you or anyone else had the AUC sensor itself go bad? Curious about the typical repair costs for that scenario.

I completely get your concern about those BMW repair costs! After your reply, I'm even more convinced we had the same issue. The error code 804A21 is exactly what I got with my 7 Series. And yeah, that humidity connection is a dead giveaway, mine also acted up like crazy when it was damp outside. I actually did end up having to replace both the wiring harness and the AUC sensor itself about a year later, the wiring fix was just a temporary solution in my case. When the sensor itself went bad, it was roughly 380 Euro for the part plus labor. Not exactly pocket change, but way better than some other BMW repairs I've had to deal with! Quick tip though, if you're getting this fixed, make sure they check the ventilation flaps while they're at it. Sometimes they can get stuck and cause similar symptoms. Mine were a bit gunked up and needed cleaning when they did the sensor replacement. By the way, how's the AC performance in general? Just curious if you've noticed any other climate control quirks besides the blower and smell issues?

Thanks for sharing those details! Yeah, the price for the AUC sensor replacement isn't as scary as I feared, honestly was bracing myself for something closer to 1000. The AC is actually working fine otherwise, just those annoying smell and blower issues I mentioned. Your tip about the ventilation flaps is super helpful. I'll definitely have them check that when I take it in. Since you dealt with both the wiring and sensor issues, I'm thinking I might just bite the bullet and get both replaced at once to avoid multiple trips to the shop. Better to sort it properly the first time around than deal with this hassle again in a few months. Really appreciate you sharing your experience, gives me a much clearer picture of what I'm dealing with. I'll book it in for next week and make sure they do a thorough check of the whole system. Thanks again for all the info!