Thermostat

Description and Referals

The "open too soon" type failure mode is included in the on-board diagnostics. The check engine light will not be lit by an "open too soon" condition. If thermostat has failed 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 DTC is present, see Cooling System Diagnosis for other probable causes. Failing shut is the most common mode of failure, and normally, only on high mileage vehicles. The temperature gauge will indicate a failed closed failure. Refer to Diagnosis and Testing and/or Diagnostic Charts.