Drain the coolant to a level below the sensor and turn it out.
Suspend the sensor on a thread in a glass vessel filled with water and connect the ohmmeter probes to the connector pin and the sensor housing. Check the water temperature with a thermometer.
Heat the water to 80°C, the sensor resistance should be within (53,5±4,2) Ohm.
If the sensor resistance is out of specification, replace the sensor.
Install the sensor in place and fill the coolant into the cooling system.