Thermostat: Testing and Inspection

THERMOSTAT

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, Component Tests and General Diagnostics 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.