Engine Thermostat

The thermostat is operated by a wax filled container (pellet) which is sealed so that when heated to a predetermined temperature, wax expands enough to overcome closing spring and water pump pressure, which forces valve to open. Coolant leakage into pellet will cause a thermostat to fail open. DO NOT attempt to free up a thermostat with a screwdriver.

The open too soon type failure mode is included in on-board diagnosis. The check engine light will not be lit by an open too soon condition. If it has failed to open, a Diagnostic Trouble Code (DTC) will be set. DO NOT change a thermostat for lack of heat by gauge or heater performance, unless a code is present, see COOLING SYSTEM DIAGNOSIS  for other probable causes. Failing shut is normal long term mode of failure, and normally, only on high mileage vehicles. The temperature gauge will indicate this.