Did you bleed up at the thermostat? Small Allen bolt there I've had air there that caused all manner of fun with the sensor iirc
Failing that have a look round for a coolant leak I've had a few on the pipe from thermostat to heater matrix, likes splitting on the clip and collects ontop of the gearbox
Failing that have a look round for a coolant leak I've had a few on the pipe from thermostat to heater matrix, likes splitting on the clip and collects ontop of the gearbox