Thermostat: Testing and Inspection

DIAGNOSIS AND TESTING - ENGINE COOLANT THERMOSTAT

Thermostat diagnostics is included in 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" malfunction occurs. Do not change a thermostat for lack of heater performance or temperature gauge position, unless a DTC is present. Thermostat remaining shut is usually the long term mode of malfunction, and normally, only on high mileage vehicles. The temperature gauge will indicate this.