SMAX automatic temperature control failure

Sounds familiar. I had a similar problem with my old Vauxhall Astra. Climate control went haywire. Did you check the interior temperature sensor? Mine turned out to be a faulty connection.

Where is the interior temperature sensor located, and how difficult is it to check the connections? Is it like checking the oil level?

It's usually behind a small grill somewhere on the dashboard. I'm no expert, but I'd recommend taking it to a workshop, as the sensor diagnostics can be tricky.

Thanks for the advice. I took it to a local workshop, and you were spot on! It was a defective cable/plug connection to the interior temperature sensor. Repair cost was 95€. Everything is working fine now!