Remove the coolant temperature sensor.
Dip the sensor element in hot water and measure the resistance.
Rated value 2.1-2.7 kΩ (at 20°C); 0.26-0.36 kOhm (at 80°C).
If the resistance value is out of specification, replace the sensor.
After installing the coolant temperature sensor in place, tighten it and connect the connector to the sensor.