Citroen C5 Overheating Slow Warmup

I had almost the exact same issue with my 2014 Opel Insignia. Overheating and slow warm-up times are a real pain. In my case, it turned out to be the thermostat getting stuck. Did you notice if the cooling fan runs constantly, even when the engine is cold? That was a big clue for me.

That's interesting about the cooling fan. I'll check that tomorrow morning. So, you're saying the thermostat itself was the problem, not necessarily the temperature sensor or water pump? Was there any coolant leak?

Yep, in my case, it was definitely the thermostat. No coolant leaks, and the temperature sensor seemed fine. Since you've already eliminated the obvious leaks, a sticky thermostat is a prime suspect. Given the symptoms and your experience, it's probably best to get it checked out by a workshop. They can diagnose it properly and replace the thermostat if needed.

Thank you so much for your advice. I've taken it to my mechanic, and you were spot on – it was indeed the thermostat. The defective map thermostat was replaced, and the cooling system is now functioning perfectly. The repair set me back 350€, but it was worth it to have the engine running smoothly again.