Diagnosing Thermostat Failure

The thermostat is a simple part but its failure affects the entire vehicle. A stuck-closed thermostat causes overheating. A stuck-open thermostat causes undercooling. Both create driveability and comfort complaints. The thermostat is cheap — usually 10 to 30 dollars — but misdiagnosing a thermostat concern and replacing expensive parts that are not faulty costs real money and credibility. Confirm the thermostat is the problem before replacing anything else.

The engine temperature climbs steadily past normal operating range. The upper radiator hose is cold or barely warm even though the temperature gauge shows the engine is hot. This is the telltale sign — the thermostat is blocking coolant flow to the radiator, so the hose on the radiator side stays cold while the engine side is boiling. Compare the hose temperature to the gauge reading. If the gauge says 230 degrees or higher and the upper hose is cool to the touch — the thermostat is stuck closed. Do not continue running the engine. Shut it off. Replace the thermostat before driving the vehicle. Sustained overheating warps the cylinder head and blows the head gasket — a 25-dollar thermostat failure turns into a 2,000-dollar repair if you ignore the gauge.

The engine takes an unusually long time to warm up. The temperature gauge barely reaches the middle or never gets there, especially in cold weather. The heater blows lukewarm or cool air. The PCM sets a P0128 code — coolant temperature below thermostat regulating temperature. This code specifically means the PCM expected the engine to reach operating temperature within a certain time and it did not. The PCM is telling you the thermostat is not doing its job. Fuel economy drops because the engine stays in a cold enrichment strategy — extra fuel to compensate for what the PCM thinks is a cold engine. The transmission may also shift differently because transmission shift strategy uses coolant temperature as an input.

Use the scan tool to monitor coolant temperature data in real time. Start with a cold engine — the temperature should match ambient. Watch the temperature climb as the engine warms. On a properly functioning thermostat, the temperature rises steadily until it approaches the thermostat rating — usually 195 to 205 degrees — then the temperature may dip slightly as the thermostat opens and hot coolant mixes with the cooler coolant in the radiator. The temperature then stabilizes in the normal range. If the temperature climbs past normal without any dip or stabilization — stuck closed. If the temperature rises very slowly and never reaches the normal range — stuck open. Some scan tools also show a calculated thermostat status — open or closed — based on temperature rate of change. Compare to the actual temperature data to confirm.

If you remove the thermostat, you can bench test it. Suspend the thermostat in a pot of water with a thermometer. Heat the water slowly. The thermostat should begin to open at its rated temperature — stamped on the body — and be fully open about 15 to 20 degrees above that. If it does not open at the rated temperature, if it only opens partially, or if it is already stuck open at room temperature — it is failed. Replacement thermostats are inexpensive. When installing, make sure the sensing element — the spring and wax pellet end — faces into the engine toward the hot coolant. Installing it backward prevents it from sensing temperature correctly.