BMW M3 Climate Failure: Multiple System Issues

Had exactly the same problem with my 2015 BMW M4 last winter. The symptoms were identical, foggy windshield, AC not working, and dead mirror heaters. Turned out the wiring harness connecting the climate control module was corroded and had loose connections. Took it to my mechanic who diagnosed it with proper BMW diagnostic tools. It wasn't just the fog sensor, the entire connection needed to be replaced and rewired. The work took about 4 hours. This isn't a quick-fix job as they need to access components behind the dashboard. It's definitely worth getting checked out properly as these symptoms can indicate more serious electrical issues if left unattended. In my case, the corrosion was starting to affect other systems too. Don't try DIY fixes on this one, the climate control system in these models is quite complex and integrated. A certified BMW technician should handle it to avoid causing additional problems.

Thanks for sharing your experience! This sounds exactly like what I'm dealing with. Really helpful to know it was the wiring harness in your case. Would you mind telling me how much you ended up paying for the fix? Also curious if you've had any related issues pop up since getting it repaired? Mine's been giving me such a headache, so I'd love to know what to expect.

Thanks for asking about the follow-up! Happy to report that since getting it fixed, everything's been working perfectly, no more fogging issues or any other related problems. My repair was actually much simpler (and cheaper!) than initially expected. The mechanic found it was just a faulty cable connection to the fog sensor that needed fixing. Total bill came to 95€, which was a huge relief since I was worried it might need the entire wiring harness replaced like they first suspected. Honestly, the repair took less than an hour once they identified the actual problem. All the climate control functions are back to normal, AC kicks in properly, mirror heaters work great, and those washer nozzles are functioning as they should. My advice would be to get it checked out soon, turns out these issues can really pile up if left alone. If you're lucky, it might be something as simple as mine was rather than a full wiring harness replacement. At least now you know what to expect. Let me know how it goes with yours!

Just got back from the mechanic and wow, what a journey this turned out to be! My issue was actually more complex than yours. Turned out the wiring harness was completely shot and had damaged the climate control module too. The repair took almost a full day, and my wallet definitely felt lighter afterward, about three times what you paid. But honestly, I'm just relieved it's fixed now. The mechanic I found was super thorough and even showed me the corroded parts they replaced. Everything's working like a charm now, no more foggy windows, AC's blasting cold air, and those pesky mirror heaters are finally doing their job. Still, really grateful for your input earlier. It helped me know what to expect and what questions to ask at the shop. Definitely learned my lesson about getting these things checked out sooner rather than later. Thanks again for helping me sort this out!