Temperature gauges show the relative temperature of a certain piece of machinery. In cars, the temperature gauge shows the temperature of the engine, which is very important for judging engine performance. In homes, temperature gauges may be used to show the temperature of water heater, refrigerators or other devices. A gauge will only show the temperature, while other devices like the thermostat will actually sense the temperature. Still, the gauge is a useful tool to diagnose immediate problems will the entire system.
If the temperature gauge has stopped working completely, first check the fuse for the temperature gauge circuit. This is mostly true in cars, since a blown fuse in a home would be more noticeable, but sometimes a single bad fuse in a car can cause problems with the temperature gauge while being difficult to spot when watching other system indicators. Check the fuse first, and replace it if necessary.
Wiring and voltage problems are a very common problem with temperature gauges. Faulty wires or voltage differences may cause the gauge to constantly display maximum temperatures, or to not work at all. If the fuses are correct, use an ohmmeter to test the wiring to make sure it is not faulty.
If your temperature gauge is fluctuating wildly, showing different temperatures with no relation between readings, then your electrical connections could be the problem. Clean the connections carefully and tighten them to make sure they are properly carrying the current.
The gauge may simply be responding to faulty readings given off by the thermostat. If the thermostat has failed, then it needs to be replaced in order for the gauge to function correctly. Thermostat sensors on engines can sometimes become disconnected and will need to be reattached to give proper readings. At other times, thermostat wires can short or become faulty, and should be tested with an ohmmeter.
If your temperature gauge consistently reads high, it could be completely correct and indicating a serious problem with your engine coolant system. This could point to low levels of coolant, in which case you will need to refill your coolant tank to the proper levels. The water pump that circulates the coolant could have broken, leading to an absence of coolant in the engine. Also, the coolant lines may have become clogged and should be checked to see if they need to be cleaned or replaced.