Thermostat: how it works, symptoms, problems, testing

May 29, 2017
Thermostat housing
Thermostat complete with the housing. Click for larger photo

The thermostat is an important part of the vehicle cooling system. The vehicle cooling system is filled with liquid coolant (antifreeze) that transfers the heat from the engine into the radiator to cool the engine down.
The thermostat helps the engine to warm up faster and maintain the proper operating temperature.

How the thermostat works: When the engine is started cold, the thermostat is closed, preventing the coolant flow through the vehicle radiator. When the engine is fully warmed up, the thermostat opens, allowing hot coolant to circulate through the radiator where it's cooled down by the passing air.

Symptoms of a bad thermostat

Symptoms of a bad thermostat can be split in two cases:
1. When the thermostat is stuck open, the engine takes too long to warm up. The engine temperature drops when driving on the highway or in cold weather. This often results in lack of heat from the heating system. The Check Engine light may come on too. Check Engine light codes related to a thermostat are P0128 and P0126.
2. When the thermostat is stuck closed, the engine overheats. Besides these two cases, the thermostat may need to be replaced if there are coolant leaks originating at the thermostat housing.

How the thermostat is diagnosed

The thermostat is not very expensive part, however if it is stuck in closed positions the engine may overheat. Overheating the engine may result in internal damage to an engine head gasket and other components. Considering this, whenever there is a suspicion of a bad thermostat, it's usually replaced.

Technican check the temperature of the cooling system
Checking the temperature of the top rad hose
Technican check the temperature of the cooling system
The bottom rad hose is warm, the thermostat is open.

Mechanics may use an infrared temperature gauge to diagnose a stuck-open or stuck-closed thermostat.
If the thermostat is stuck open, the coolant is flowing through the radiator even when the engine is cold. This will result in the radiator and both radiator hoses to become warm soon after the engine is started.
If the thermostat is stuck closed, there is no flow from the engine through the radiator, even when the engine is fully warmed up. This will result in large difference in temperature between the engine and the bottom part of the radiator. The top hose might be warm or hot, but the bottom hose will remain cold or barely warm.

When the thermostat is working correctly, there is no flow through the radiator when the engine is started cold. The radiator remains cool. Once the engine is fully warmed up (after 10-20 minutes), the thermostat opens and coolant flows openly through the radiator. The top radiator hose is hot, while the bottom hose is warm.
For example, in this fully warmed-up Honda in the photos, the engine is at 182 ℉, the top radiator hose is at 134 ℉, while the bottom hose is at 94 ℉. The radiator fan comes on, runs for a while and comes off. This means the thermostat is currently open. It was closed when the engine was started. The thermostat in this car works correctly.

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Thermostat replacement

Depending on the vehicle replacing the thermostat may cost from $120 to $350. Whenever the thermostat is replaced, the engine is usually refilled with fresh coolant (antifreeze).
Should the thermostat be replaced as a part of regular maintenance? Thermostat replacement is not included in scheduled maintenance. It's a good idea to replace a thermostat if the car was sitting for very long time without driving or whenever the engine is rebuilt or replaced.
Some manufacturers recommend reflashing the engine computer when there is a Check Engine code related to the thermostat, after it has been replaced.

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