The ECT sensor is also known as an Engine Coolant Temperature sensor. As its name implies, the primary function of this sensor is to monitor the temperature of your engine's coolant. In so doing, the ECT can better understand how hot (or cold) your engine is, simply by the heat that the coolant is giving off. With this data and input, the ECT sensor could then inform your car's central computer (the ECU) on how to best manage your engine's cooling system. For example, if the ECT is reading an overly high temperature, the ECU might take active measures to help cool your engine. While it doesn't directly control the circulation of the coolant, the ECT sensor's input might prompt the ECU to, for example, ramp up the radiator fans to cool the engine a bit better.