Examination
1. If the coolant temperature sensor is not working, check its fuse.
2. If the sensor indicator shows that the engine is hot when you first started a cold engine, disconnect the wire from the sensor (indicated by an arrow). If the indicator reading decreases, replace the sensor. If the readings remain the same, then the indicator wiring is broken, or the indicator is faulty. In the picture on the left is a 4-cylinder engine, on the right is a 6-cylinder.
3. If the indicator does not show anything, after the engine has warmed up and the indicator fuses are intact, turn off the engine, disconnect the wire from the sensor and use a jumper wire to connect the wire to engine ground. Without starting the engine, for a moment, turn on the ignition. If the indicator shows that the engine is hot, replace the sensor.
Replacement
1. Drain the coolant.
2. Disconnect the wire from the sensor.
3. Unscrew the faulty sensor and screw in the new one. Connect the wire and check the operation of the sensor.