Mini One heating blowing lukewarm air

Sounds like a faulty water valve, or possibly a blockage in the coolant system. I had a similar issue with my Mini Cooper a while back. The car heating was barely working, and the engine was running hotter than usual. Have you checked for any signs of a coolant leak? Also, is there an error code stored in the car's computer? That can sometimes give you a clue. Could be a simple fix like a corroded plug connection on the valve itself.

Coolant leak - no. Error code - yes, something about the water valve. You mentioned a corroded plug connection - where exactly is that located? Is it something I can easily check myself without taking everything apart?

Yep, the error code pretty much confirms it’s the water valve. The plug connection is right on the valve itself. It controls the flow of engine coolant to the heater core that's the little radiator inside your dashboard that provides hot air. It's worth a look, but honestly, if you aren't too familiar with car electrics, you might be better off taking it to a workshop. They can properly diagnose it and replace the valve if needed.

Thanks a bunch for confirming that! I took it to a mechanic, and you were spot on. Turns out the water valve was indeed defective. They replaced it, and the car heating is blasting hot air again. 95€ well spent!