Engine Thermostat Testing

THERMOSTAT
The thermostat is operated by a wax filled chamber (pellet), which is sealed. When heated coolant reaches a predetermined temperature the wax pellet expands enough to overcome the closing spring and water pump pressure, which forces the valve to open. Coolant leakage into the pellet will cause a thermostat to fail open. Do not attempt to free up a thermostat with a screwdriver.
Thermostat diagnostics is included in Powertrain Control Module's (PCM) programming for on-board diagnosis. The Malfunction Indicator Light (MIL) will illuminate and a Diagnostic Trouble Code (DTC) will be set when an "open too soon" condition occurs. Do not change a thermostat for lack of heater performance or temperature gauge position, unless a DTC is present. Refer to Diagnosis for other probable causes. Thermostat failing shut is the normal long term mode of failure, and normally, only on high mileage vehicles. The temperature gauge will indicate this. Refer to Cooling System Diagnosis.