To ensure the most accurate results, the engine coolant temperature sensors have their tips immersed in coolant. It works quite simply by measuring the temperature of this coolant. Thus, allowing it a better understanding of how hot or cold your engine is. The ECT (engine coolant temperature) sensor then passes along all that data to the ECU (your car's computer brain). It does so by sending the ECU electric signals, corresponding with the temperature of the coolant. Initially, the ECU sends the ECT sensor a 5V (typically, though it might differ) reference voltage. The changes in voltage will let the ECU know the coolant's temperature. From there, the ECU can take active steps to help cool down (or heat up) your engine, such as ramping up the radiator fans or using a richer fuel mixture.