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

The coolant thermostat plays a critical role in an engine's cooling system by regulating the engine's operating temperature. It opens and closes based on the coolant temperature, allowing coolant to flow to the radiator when the engine is too hot and restricting flow when it’s at optimal temperature. This mechanism ensures that the engine operates efficiently and does not overheat.

The other options mentioned, such as showing the engine's operating temperature or coolant level in the radiator, do not accurately reflect the thermostat's primary function. Instead, the thermostat's regulation is essential for maintaining optimal engine performance and longevity. Understanding this is crucial for vehicle maintenance and safety on the road, following German traffic laws that emphasize vehicle reliability and safety standards.