The coolant temperature indicator system has a light or temperature gauge in the instrument panel and a coolant temperature sending unit mounted to the engine. The car may have more than one coolant temperature sending unit, but only one is utilized in the indicator system. If the light or gauge signals a high temperature, check the coolant level and ensure wiring on the light or gauge and sending unit is secure along with checking fuses. When the key is turned to the right and the starter motor is engaged, the lamp should light up when the engine is overheated. If the light does not come on, it could be due to a burned-out bulb, a faulty ignition switch or an open circuit. Grounding the wire to the sending unit with ignition on (engine off for safety) will test the circuit. If the gauge deflects full scale or light comes on, the sending unit should be replaced. When the engine is running, the light must go out and stay out until overheating occurs. If the light doesn't go out, the wire is probably grounded, the sending unit may be defective, or the ignition switch may be to blame. Also make sure that the type of coolant is correct; plain water may have a boiling point too low to activate the sending unit. If the sending unit is defective, it can be unscrewed from the engine. A new one can then be installed along with some sealant on the threads. It is best to perform this work when the engine is cool so that less coolant spills out. The level can then be checked after installing the new one.