A test you can do is start the engine from ice cold and let it idle. Have the front bonnet up and feel the coolant pipe that disappears through the bulkhead to the radiator. If it warms up slowly, getting hotter and hotter, your thermostat is fully or partially open all the time. You would expect the pipe to stay cool then all of a sudden become hot as the hot coolant is passed through the radiator when the thermostat opens.
With the engine hot (electric radiator fan cutting in), jump in and drive on a nice open road at speed. If the coolant gauge drops to cold from mid way, you will need a new thermostat.
If the gauge never rises to the normal half way mark whilst stationary (again with the radiator fan cutting in), your gauge is not working. This is normally caused by a sensor fault or a wiring problem (bad connection between the sensor and it's connector plug).
You can test the gauge operation by disconnecting the sensors plug and connecting the wire to earth. This should make the gauge shoot to hot, confirming correct operation of the gauge itself.
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Created : 2011-10-05 15:42:50, Last Modified : 2011-10-05 15:42:50