What is the function of a coolant thermostat in an engine's cooling system?

Your answer is correct. The coolant thermostat is a key component in an engine's cooling system. Its main function is to regulate the engine's operating temperature by controlling the flow of coolant between the engine and the radiator. When the engine is cold, the thermostat stays closed so the engine can warm up quickly. Once the engine reaches the optimal temperature, the thermostat opens to let coolant flow and prevent overheating. It does not display the engine temperature or coolant level—those are the jobs of different sensors and gauges. Understanding this helps with both driving safety and basic vehicle maintenance.