So, on the last trip to France, we started to get these weird symptoms: The red coolant-level light would start flashing and the temperature gauge would go unnaturally quickly to maximum and stay there. Stopping the engine and re-starting would often seem to cure the problem.. the temperature gague would return to normal and the flashing light would not show apart from a few seconds at startup.

Firstly, I verified the coolant level was ok. I was worried for a bit that there might be air caught in the radiator system.  There are lots of long pipes making it nice and  complex, with the radiator in the front and the engine and water pump in the back. That was all fine, however, I topped off the coolant tank and the expansion/inspection tank as well. Nothing wrong there.

Then, it was time to look at the wiring diagram and do some experimentation, which revealed:

• The flashing light is latching, in that it will stay on until the ignition is turned off. So that’s why stopping and restarting the engine sometimes cured the problem.
• In my case, the temperature gauge going to maximum is something to do with the red light, not the temperature itself. The wiring diagram sort of suggested this, but I proved it by disconnecting the cable from the sensor. Once open circuit, the flashing light goes on and the temperature gauge goes to maximum. Ok, we are getting somewhere here.
• So, given there is not evidence of missing coolant or overheating, I suspect either the sensor, the connection or the wiring to the sensor.

Cleaning the contacts on the connections to the sensor seems to have solved the problem. The flashing light comes on for 5 seconds after the ignition is turned on, then goes off and stay off. Excellent.