Mercedes Viano Diesel AC Problem

Had similar on my Vito. Check the pressure switch first, it's a common failure. If the system doesn't detect enough refrigerant pressure, it won't engage the compressor. Do you have access to an AC manifold gauge set to check the system pressure?

I don't have a manifold gauge set. I'd have to buy one. Is there any way to test the pressure switch itself without special tools? What does the AC relay do? Could a faulty AC relay also cause the same issue?

You can try bypassing the pressure switch to see if the compressor kicks on, but be careful. If there's actually low refrigerant, you could damage the compressor. The AC relay is what sends power to the compressor clutch. If it's bad, the compressor won't engage. Honestly, without proper AC diagnostics tools, you're mostly guessing. Considering the symptoms, it's probably best to take it to a workshop to check the valves, like you initially suspected. I had the same issue and it turned out to be faulty valves.

I took it to a local mechanic. They confirmed it was a valve issue within the air conditioning system. The repair cost was exactly 1000€. At least it's fixed now.