If the valve is open, replace the thermostat with a new one.
To check vaive opening temperature, suspend the thermostat [A] in a container of water and raise the temperature of the water.
The thermostat must be completely submerged and must not touch the container sides or bottom. Suspend an accurate thermometer [B] in the water. It must not touch the container, either.
If the measurement is out of the specified range, replace the thermostat with a new one.
Thermostat valve opening temperature: 58-62°C (136-144°F)