What Does a Thermostat Do in a Car?

Close up of a car's temperature gauge.
A thermostat reduces engine wear, deposits, and emissions by letting the engine warm up quickly. Jackyenjoyphotography / Getty Images


A car is intricately constructed, with each and every component serving a vital role in it's performance and longevity. With that in mind, those asking, "what does a thermostat do in a car," may be surprised to discover it's true value.


­Designed to monitor and maintain optimal engine temperature, the thermostat in most cars is only about 2 inches (5 cm) in diameter. It may be small, but it plays a vital role in preventing an engine from running too hot or too cold.

How a Thermostat Works

Every liquid-cooled car engine has a small device called the thermostat that sits between the engine and the radiator. It serves as a valve to control coolant flow, and is designed to open and close at specific temperatures.

When the engine is cold, the thermostat remains closed, blocking the flow of coolant to the radiator, allowing the engine to warm up quickly to its optimal temperature [source: Ofria].


Once the engine's temperature reaches the desired temperature (generally about 200 degrees F, 95 degrees C), the thermostat opens, permitting the flow of coolant to the radiator, and preventing the engine from overheating.

The secret of the thermostat lies in the small cylinder located on the engine-side of the device. This cylinder is filled with a wax that begins to melt at perhaps 180 degrees F (different thermostats open at different temperatures, but 180 F/82 C is a common temperature).

A rod connected to the valve presses into this wax. When the wax melts, it expands significantly and pushes the rod out of the cylinder, opening the valve. If you have read How Thermometers Work and done the experiment with the bottle and the straw, you have seen the same process in action. The wax happens to expand a good bit more because it is changing from a solid to a liquid in addition to expanding from the heat.


Why a Thermostat is Important

A functional thermostat is essential for a number of reasons, including:

  • Enhanced performance: By maintaining the engine at its ideal temperature, the thermostat ensures that the engine runs efficiently [source: Hawley]. This leads to smoother performance and can even boost fuel efficiency.
  • Reduced emissions: engines that operate within their optimal temperature range produce fewer emissions. This is because the combustion process is more complete, reducing the release of harmful pollutants.
  • Prevention of overheating: Overheating is one of the most common causes of engine damage. The thermostat helps prevent this by ensuring that coolant circulates through the engine when it reaches high temperatures.
  • Prolonged engine life: By preventing extreme temperatures, the thermostat contributes to the longevity of the engine and its components. This can save car owners significant money in repair and replacement costs over time.


Symptoms of a Faulty Thermostat

A malfunctioning thermostat can lead to various issues, including engine overheating, under heating, or erratic temperature fluctuations [source: Cooper]. Some signs that may indicate a faulty thermostat include:

  • Engine overheating: If the thermostat is stuck in the closed position, it prevents coolant from circulating through the radiator to be cooled, causing the engine temperature to rise rapidly. This overheating can cause serious damage including warped cylinder heads, blown head gaskets, and in severe cases, complete engine failure.
  • Engine running too cold: If the thermostat is stuck open, it allows coolant to flow freely through the engine and radiator, preventing the engine from reaching its minimum operating temperature. An engine that runs too cold is inefficient, leading to poor fuel economy.
  • Fluctuating engine temperatures: Fluctuating engine temperatures could be the result of a thermostat that's opening and closing irregularly. This could in turn confuse the engine's computer, leading to poor performance, reduced fuel efficiency, and increased emissions.
  • Unusual temperature gauge readings: The temperature gauge on your car's dashboard can be the first sign of a faulty car thermostat. Unusual readings or significant fluctuations should prompt a check of the thermostat and the cooling system.
  • Increased fuel consumption: When the engine doesn't reach optimal temperature, it can run less efficiently, requiring more fuel to maintain performance.

Recognizing the symptoms of a faulty thermostat is essential for maintaining your vehicle's health and performance. If you experience any of these issues, get your vehicle examined by a professional. Early diagnosis and replacement of a faulty thermostat can prevent more serious engine problems.


Engine Temperature 101

The thermostat may be small, but it remains an indispensable component of modern cooling systems. By regulating the engine's temperature, it ensures optimal performance, reduces emissions, and prevents damage.

Regular maintenance and timely replacement of a faulty car thermostat are essential practices for any car owner looking to preserve their vehicle's health and efficiency.


Car Thermostat FAQs

What are the telltale signs of a failing thermostat?
Signs include very high temperature readings, an overheating engine, leaking coolant and erratic changes in temperature.
What should I expect to pay to replace my car thermostat?
On average, a professional mechanic will charge you $140-$300. It's safe to assume that this estimate will vary from car to car.
What will happen if I drive my car without a thermostat?
A car without a thermostat will not let the engine reach optimal operating temperature. It may also result in the heater giving off cold air.