The thermostat is a small piece of equipment in your car's engine that plays an important role. It's no longer than the size of an adult finger and looks a bit like a peg with a spring around it. It's located in the top radiator hose on the engine block in most vehicles, although some have it built into a housing that sits on or near the water pump.
Thermostats are simple devices but they're quite effective. They consist of a small cylinder filled with a wax that starts to melt at perhaps 180 degrees F (the temperature can vary significantly from vehicle to vehicle). As the wax begins to melt it expands, pushing on a rod connected to it and opening a valve to allow coolant to flow in.
Coolant is then able to flow around the engine, absorb heat, and be passed back through the radiator. As the coolant is cooled it will begin to return to its starting temperature, and the thermostat will close again.
If your thermostat fails to open it won't be able to control the engine temperature, and the engine will start to overheat as it tries to warm up. It will also take a long time to reach the proper operating temperature, which can cause problems with other components in your cooling system like the radiator and gaskets.
Thermostats rarely fail, but they can go bad due to age, corrosion, or damage. If you need a new one, it's worth having a professional replace it to ensure that your car can operate properly and avoid overheating or other cooling system problems.