Seat Toledo heating blowing lukewarm air

Sounds like a water valve issue, I had something similar with my Toledo. Is there coolant leaking? Check the carpet on the passenger side; it could indicate a heater core leak, although that's less likely. It could also be a valve blockage. Have you checked the engine coolant level?

Coolant level seems OK, no visible leaks, and the carpet is dry. Where exactly is this water valve located? And what does it do exactly?

The water valve controls the flow of engine coolant to the heater core, like a tap controlling water flow to a faucet. If it's faulty, not enough hot coolant gets to the core, and you get poor heating. It's a common problem. If the error memory points to it, and you've checked the obvious things, I'd say it's highly likely that's the culprit. It's probably best to get it checked out at a workshop.

Thanks a lot for your help! I took it to the workshop, and you were right. It was the water valve. They replaced it, and now the heat is working perfectly. The bill was 95€, which is fine. I am so happy!