The Engine Coolant Temperature Sensor (CTS or ECT) uses electrical resistance to measure the temperature of the coolant or antifreeze mixture in the cooling system. This gives an indication of how much heat the engine is giving off. The vehicle’s computer will use this information to adjust certain engine functions to keep engine temperature at an ideal level. It may turn the cooling fan off or on, open the exhaust gas recirculation or communicate the need for a richer fuel mixture.