An automotive thermostat is a kind of a temperature-sensitive valve that allows or restricts coolant to flow through the radiator. It helps an engine to warm up faster, while preventing it from overheating. By design, it is a valve or switch located somewhere between your engine and the radiator, which opens when the coolant temperature reaches a specific set point, and closes when the temperature drops to below it. In its closed position the coolant circulates through the engine to help it warm up as fast as possible. However, once the engine is getting overheated the thermostat opens, redirecting the coolant to the radiator, so the latter one can cool down the coolant and return it back to the engine. 

While most thermostats are mechanical temperature-operated valves, some of their newer counterparts are computer- or map-controlled devices. These electronic thermostats are operated by an engine control unit which opens or closes the thermostat based on the data received from different sensors. An electronic thermostat can also blend hot and cold coolant to keep the engine running within the most comfortable temperature range. This increases engine performance, reduces emissions and improves fuel economy. 

When the engine is running, your thermostat must quickly and properly respond to temperature changes in the engine. If it fails to do that or malfunctions, you’ll face a variety of troubles, including engine overheating, an illuminated check engine light, etc. Here is what can happen if your thermostat gets stuck open or closed.

Your thermostat is stuck open

If your thermostat gets open all the time, the coolant will flow through the radiator even when this is not needed. As a result, you’ll notice the following problems:

  • Low readings on your temperature gauge
  • Increased fuel consumption
  • Problems with your heater
  • Hard shifting into higher gears if you have an automatic transmission

Most of them are not critical and will not ruin your engine, yet may affect your comfort and fuel consumption. 

Your thermostat doesn’t open

This is the worst case scenario that may happen. If the thermostat sticks closed when the engine is extremely hot, running at full throttle, you may experience serious engine problems:

  • Your engine can overheats and gets severe damage
  • Leaking head gasket
  • Steam or smoke from under the hood
  • Illuminated check engine light and warning lights

Driving with a closed thermostat is a perfect way to ruin an engine. That’s why once you notice any of these signs, you should shut down the engine and get in touch with your mechanic.

The evolution of a thermostat has turned it into a reliable device that can serve you for years to come, however, there are still several factors which can shorten its service life:

  • Rust, corrosion and deposits
  • Physical damage that will cause leaks
  • Electronic malfunction for computer-controlled thermostats 

How to fix a bad thermostat

The answer is very simple. All you need to do is to replace a bad thermostat with a new one as it is not repairable. Unless you are a mechanic, it’s better to leave this job to a professional, who will not only replace your bad thermostat, but also inspect the cooling system for any possible problems. 

Replacing a thermostat involves installing new gaskets to prevent coolant leaks. It’s also recommended to replace a thermostat whenever a water pump or radiator is replaced. For vehicles equipped with an electronic thermostat, you’ll need a special scan tool plugged into the OBD-II port to read the trouble codes.

Ignoring signs of a bad thermostat will lead to time-consuming repairs that will cost you a pretty penny. It is much cheaper and easier to prevent severe engine problems by replacing a bad thermostat than having to deal with expensive engine repairs.

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