First of all, the coolant temperature indicator system consists of a gauge on the dashboard and a sending unit on the engine. Gasoline engines have a temperature-sending unit on the intake manifold and a separate CHT sensor for the PCM. Also, diesel engines use an ECT sensor for both the PCM and gauge. To troubleshoot overheating, check the coolant level, wiring connections, and fuses. Test the circuit by disconnecting the sender and connecting a jumper wire. If the gauge doesn't read full HOT, there's an issue in the circuit. For diesel engines, have the ECT sensor checked by a dealer. To replace the sending unit, prepare the new unit, release system pressure, disconnect the connector, unscrew the old unit, and install carefully as to not lose coolant.