robbie-rocket-pants wrote ...

After driving around with the sensor disconnected for a few days there was no more recurrence of the fault warning so I concluded it was not a wiring short and the fault must be with the sensor.

I then set about fettling with the sensor:

1. I syphoned off the contents of the expansion tank (about a pint)

2. Turned the sensor through 90 degrees and wiggled it out of the expansion tank

3. I noticed 2 prongs on the sensor which I presume must allow a small electric current to flow when dipped in the coolant, but instead of being shiny metal they had a coating on them (oxidation maybe)

4. So I got a bit of sandpaper and cleaned the prongs back to shiny metal

5. I put the sensor back in (starting at 90 degrees then pushing and turning), re-connected it, re-filled the expansion tank and hey-presto !

Fixed for tuppence ha'penny.

Hope this helps someone.

Cheers, RRP.

Here are some photos.