Diagnosis And Testing

The "open too soon/stuck open" type failure mode is included in the on-board diagnostics. The check engine light will not be lit by this condition. If thermostat has failed open, a Diagnostic Trouble Code (DTC) will be set (P0128). Do not change a thermostat for lack of heat by gauge or heater performance, unless a DTC is present, for other probable causes, refer to DIAGNOSIS AND TESTING . 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.