Which of the following is NOT a function of the thermostat?

The thermostat in an engine cooling system primarily serves to regulate the engine's temperature by opening and closing in response to coolant temperature changes. It ensures that the engine reaches its optimal operating temperature efficiently by allowing coolant to flow to the radiator or bypassing it when the engine is still cold.

Regulating coolant temperature is a main function, as it helps maintain the engine within a specific temperature range. Additionally, allowing coolant to bypass is essential for preventing thermal shock and achieving quicker warm-up times when starting from cold.

However, controlling the water pump speed is not a function of the thermostat. The thermostat does not directly manage how fast the water pump operates; that function is typically managed by the engine's electronic control unit or additional components in the cooling system. Therefore, the assertion that controlling water pump speed is a function of the thermostat is not accurate.