Temperature sender units are common to become faulty, especially fard and vauxhall's
Probably best to get it checked professionally, you could check out the sponsers page as there are several garages listed there.
There is a easy way to test the operation of the gauge and the wiring.
- Locate the temperature sender unit on your engine (Normally close to the thermostat area)
- Remove the wire from it (Typically there is just one wire)
- With the ignition switched on the gauge should read cold.
- Then put the wire to earth (Chassis or engine should do) and the gauge should go all of the way up to the maximum reading (Hot)
If this happens then the gauge is operating correctly and the wiring is fine so it has to be the temperature sender unit.
The temperature sender units normally screw in easily, but beware as some have a tapered thread to allow for a good seal so DO NOT Overtighten it as you will break it
Some newer vehicles have several temperature senders, only one of them sends the signal to the temperature gauge inside the vehicle, the other ones are to send signals to the E.C.U.