"It has a water temp gauge which, until recently, was working well. Recent track day and it would just show top of the gauge. Have been advised to try replacing sensor but I don't know where the gauge takes the signal from. It is a standard caterham gauge." You can track (!) down where the fault is by turning the ignition on, disconnecting the wire from the sensor (once you've found it), shorting it to earth on the block and seeing what the gauge does. Jonathan