The coolant temperature indicator system includes a dashboard light or gauge and a sending unit on the engine. Some vehicles have multiple sending units, but only one is used for the indicator. If the car has an electric cooling fan, avoid the fan blades as they can start unexpectedly. If overheating is indicated, check the coolant level, ensure secure wiring between the light/gauge and sending unit, and check all fuses. To test, ground the wire to the sending unit with the ignition on. Once the engine starts, the light should go out unless the engine overheats. If it doesn't, it could be a grounded wire, defective sending unit, or faulty ignition switch. Check the coolant type, as plain water may not activate the sending unit. If replacing the sending unit, unscrew it from the engine and install the replacement with sealant on the threads. Let the engine cool before removal and expect some coolant loss. Check the coolant level after replacement.