A thermostat controls when coolant flows to the radiator. If it sticks open, the engine warms up slowly, the temperature gauge reads low, the heater blows lukewarm air and the check engine light may show P0128. If it sticks closed, coolant cannot reach the radiator and the engine overheats quickly, often within minutes of starting. A thermostat that sticks intermittently causes temperature swings.
Thermostat stuck open vs stuck closed
| Stuck open | Stuck closed | |
|---|---|---|
| Temperature gauge | Low or slow to rise, drops on the highway | Climbs fast, overheats |
| Heater | Lukewarm; see heater blowing cold air | Hot at first, then problems as it overheats |
| Radiator hoses | Both warm early | Upper hose hot, lower hose cool |
| Codes | P0128 (coolant below thermostat temperature) | Overheating codes such as P0217 |
| Risk | Fuel economy, wear, poor heat | Engine damage from overheating |
Symptoms of a thermostat stuck open
- Engine takes much longer to reach normal temperature
- Gauge sits lower than usual or drops at highway speed in cold weather
- Poor cabin heat and worse fuel economy
Symptoms of a thermostat stuck closed
- Fast overheating, sometimes with coolant boiling over
- Upper radiator hose very hot while the lower hose stays cooler
- Steam or coolant smell; see engine overheating
How to test a thermostat
- Watch coolant temperature on a scan tool from a cold start; it should rise steadily and level off at the normal temperature.
- Carefully feel the upper radiator hose as the engine warms: it should get hot suddenly when the thermostat opens.
- Removed, a thermostat can be tested in a pot of heated water with a thermometer to see when it opens.
- Rule out the coolant temperature sensor; see coolant temperature sensor.
Thermostat stuck partly open
Thermostats do not always fail fully open or closed. One that opens late or only partly can cause the gauge to climb in traffic and fall on the highway, or run slightly hot when towing. Temperature that swings up and down without a pattern is a classic sign. A scan tool showing coolant temperature over a drive makes these patterns easy to see.
Replacing a thermostat
Use the correct temperature rating and a new gasket or seal, then refill and bleed the cooling system; see how to bleed coolant. Air left in the system after a thermostat change is a common cause of overheating.
Stuck closed is urgent
An engine that overheats repeatedly can warp the head and fail the head gasket; see cracked engine block or warped head. Stop and let it cool rather than driving on.
More guides: outboard thermostat.
Trouble codes: P0125 Insufficient Coolant Temp for Closed Loop · P0116 Coolant Temp Range/Performance · P0126 Insufficient Coolant Temperature · all OBD2 codes.
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