Peugeot 406 Overheating and Coolant Leaks

Sounds familiar. I had similar issues with my old Opel Astra, though it wasn't overheating that badly. Turned out to be a control unit communication error. Is the engine light on?

Control unit error? How did you figure that out? And yes, the check engine light is on.

Well, the garage ran diagnostics and said there was a problem with the data link. Like a bad connection somewhere. They said it could be a software glitch or some bad wiring. If I were you I would take the car to a workshop to be checked before the engine gets any more damaged. I had a similar coolant spill.

Thanks. Took it to a shop; faulty connection like you said. 95€ and it's fixed.