Seat Altea AC problem

That sounds like a tricky issue. I am not super knowledgeable about car AC, but I did have a similar problem once with my old car. The error message came on and the HVAC system just wouldn't kick in. Have you checked for any obvious signs of a coolant leak? Maybe have a look around the engine cooling system components? It might give you some clues before you take it in for a costly repair.

That's a good point about the coolant leak. Where exactly should I be looking for this leak? I am familiar with the basic components, but is there a specific area around the heat exchanger or temperature sensor where leaks are more common? Any pointers would be greatly appreciated.

Okay, so I am no expert, but it really does sound like the temperature sensor is causing the HVAC system to not work. Given the error codes and symptoms you described, it is very likely a faulty sensor. Honestly, I would just take it to a workshop and have them confirm. They can properly diagnose the issue and replace the defective part. It will probably save you a lot of headache in the long run.

Thanks for the confirmation. I took it to a trusted mechanic, and you were spot on. The temperature sensor for the heat exchanger was indeed faulty. After replacement, the error is gone, and the air conditioning is working as expected. The entire repair, including parts and labor, set me back 980€. Appreciate your help!