Mini One Overheating

The symptoms you describe are consistent with a malfunctioning engine cooling system. Car overheating and the check engine light illuminating often point to issues with the radiator fan or its control mechanisms. Have you checked the coolant level? Is the radiator fan operating at all?

Coolant level is fine. How can I tell if the radiator fan is working properly? I mean, when should it be running? Is there a way to test the thermal switch?

Given your description, a defective radiator fan is a strong possibility. The thermal switch regulates the fan's operation, and a faulty speed sensor can cause the fan to malfunction, leading to car overheating. Due to the potential complexity and the need for specialized tools, I would advise a visit to a qualified workshop for diagnosis and repair.

Thank you so much for your help. Just got my Mini One back. It was indeed the radiator fan, probably due to a faulty speed sensor. Cost me 250€ to fix, but at least my engine isn't trying to melt itself anymore!