The pointer of the device is constantly in the red zone
With the ignition on, disconnect the wire from the sensor. If the arrow returns to the beginning of the scale, the sensor is faulty. Otherwise, either the device is faulty, or the wire is shorted to «mass». To check, remove the instrument cluster (see Removing the instrument cluster), disconnect the green wire with a white stripe from the pointer and turn on the ignition. If the arrow returns to the beginning of the scale, the coolant temperature gauge is OK, and the wire is damaged.
The pointer of the device is constantly at the beginning of the scale
With the ignition on, disconnect the wire from the sensor and connect it to «weight». If the arrow deviates, the sensor is faulty. If the arrow does not deviate, remove the instrument panel (see Removing the instrument cluster) and the output of the temperature gauge, to which the green wire is connected, is connected to «weight». If the arrow deviated, the device is working, and the wire between the sensor and the temperature gauge is damaged
You can check the sensor with an ohmmeter. Serviceable sensor at liquid temperature less than 40°С (pointer at the beginning of the scale) must have a resistance of 640–1320 ohms, at 90–100°C (beginning of the red zone on the instrument) - 77-89 ohms, and at the end of the red zone of the scale - 40-50 ohms.
The sensor is non-separable - not subject to repair.