I have a strange question. Some mornings I have the heater on and the rear window demister on cos its cold and the warning buzzer comes on.

I have checked the oil, water level, pressures and everything seems fine, what else could it be?

Maybe a silly electrical gremlin.

Time for a workshop service me thinks.

 

Have you checked the coolant level at the reservoir and the coolant neck. If so, you might want to test the sensor itself outlined in the FSM I believe.