Which component is responsible for causing the engine to overheat or run cooler than normal if it fails?

The thermostat plays a vital role in regulating the engine's temperature. Its primary function is to control the flow of coolant within the engine and the radiator. When the engine is cold, the thermostat remains closed, allowing the engine to reach a suitable operating temperature quickly. Once this temperature is achieved, the thermostat opens, allowing coolant to circulate through the engine and the radiator to dissipate heat.

If the thermostat fails and remains closed, it will prevent coolant from flowing to the radiator, causing the engine to overheat. Conversely, if it becomes stuck open, coolant flows continuously, preventing the engine from reaching its optimal operating temperature, which can lead to inefficient performance and increased emissions. Therefore, the thermostat is directly responsible for maintaining proper engine temperature, making it critical to engine performance and lifespan.