If your temperature gauge is not working it's most likely caused by a bad or faulty temperature coolant sensor.
The sensor is located either on your intake manifold or near your water pump or somewhere on your engine.
The coolant temp sensor is in contact with the coolant and when the coolant heats up the sensor senses the heat and then raises the temperature gauge.
Sometimes it may be a bad connection to the coolant temperature sensor or a bad wire.
Signs of a bad coolant temp sensor include.
Check Engine Light.
Poor Mileage.
Electrical Cooling Fans not coming on.
Black Smoke from the Exhaust Pipe.
Hard Start Condition.
Engine Overheats.
Poor Idling.
Poor Engine performance.
Most of the time the coolant temp sensor is located close to the thermostat of the cooling system, or in some cases even inside of it.
Note: Some engines may actually have a pair of sensors—one sending information from the engine to the control unit, the other sending information from the control unit to the dash.
The most common reason your temperature gauge staying on cold is a faulty coolant temperature sensor.
It can also be caused by bad wiring between the cluster or the sensor.
In some cases, it can also be a stuck thermostat causing the engine not to heat up properly.