For detailed thermostat diagnostic procedures. Refer to Coolant Heater Diagnosis .

The coolant thermostat can be tested using a temperature (tempil) stick. The temperature stick is a pencil like device. It has a wax material containing certain chemicals which melt at a given temperature. Temperature sticks can be used to determine a thermostat's operating range, by rubbing 87°C (188°F) and 97°C (206°F) sticks on the thermostat housing. The marks made by the sticks should melt when coolant temperatures of 188°F and 206°F are reached, respectively. These temperatures are the normal operating range of this thermostat, fully opened at 206°F and fully closed below 188°F. Replace the thermostat when it operates outside of this range.