C2 fog sensor and air conditioning issues

Sounds familiar. I've got a C2 too, and had a similar issue last winter. Check the cable and plug connection to the sensor itself. Seems like a weak point.

Where exactly is this fog sensor hiding? I'm more of a 'YouTube-fixes-it' kind of guy. Is it easy to get to?

That was it. But if you're not super confident, I'd suggest taking it to a workshop. It's a bit fiddly. Plus, you'll need to clear the error code afterwards anyway.

Took your advice and went to the workshop. Turns out it was a dodgy connection, just like you said. 95€ later, all sorted! Air con is blasting, mirrors are toasty, and no more annoying error messages. You saved me a headache!