What Is the Thermostat and How Does It Work?

The thermostat is an important part of an automotive cooling system. It works in conjunction with the radiator, coolant and other cooling system components to regulate the temperature of a vehicle's engine. Because it's located within a coolant hose, however, you typically won't see it -- not unless you remove the coolant hose, at least. What is the thermostat exactly, and how does it work?

What Is the Thermostat?

The thermostat is a small valve that controls the flow of coolant in response to the temperature of the coolant. Automotive engines work best at a specific temperature. If your vehicle's engine is too cold, the oil and other fluids will remain thick and sludge-like. If your vehicle's engine is too hot, on the other hand, you may experience a catastrophic problem like a blown head gasket or a warped cylinder head. The thermostat's job is to ensure that your vehicle's engine stays within the appropriate operating range.

How the Thermostat Works

While there are different types of thermostats, most of them work in the same way. They are designed to open when the temperature of the coolant reaches the ideal operating temperature. For most vehicles, this is between 180 and 195 degrees Fahrenheit. After parking and turning off your vehicle's engine, the temperature of the coolant will begin to drop. And as the coolant temperature begins to drop, the thermostat will revert back to its closed position.

If you inspect the thermostat, you may notice that it has a wax cylinder. This wax cylinder allows the thermostat to open and close in response to the coolant temperature. Heat will force the wax cylinder to expand. As it expands, the wax cylinder will push a rod that forces the thermostat to open.

Beware of a Stuck Thermostat

Thermostats are relatively simple devices that work automatically by opening and closing in response to the coolant temperature. But that doesn't mean they are immune to failure. Thermostats can still fail, and one of the most common ways in which they fail is by sticking.

Your vehicle's thermostat may become stuck open or stuck open. If it's stuck open, coolant will continue to flow through it, regardless of the temperature. This means your vehicle's engine will never warm it. The engine will remain cold, so the oil and other fluids will remain thick.

If your vehicle's thermostat is stuck closed, coolant won't be able to flow through it, regardless of the temperature. The engine will continue to heat up, and since the thermostat is closed, you may experience catastrophic heat-related damage, such as a blown head gasket or warped cylinder heat.