The Engine Coolant Temperature (ECT) sensor, sometimes referred to as the Coolant Temperature Sensor (CTS), is a thermistor (resistor which changes value based on temperature) mounted in the engine coolant stream. Low coolant temperatures produce a high resistance — 100,000 ohms at −40°F (−40°C), while high temperatures cause low resistance — 70 ohms at 266°F (130°C).
Fig. 1: Engine Coolant Temperature (ECT) sensor and connector
The control module provides a 5 volt reference signal to the sensor through a resistor in the module and measures the voltage. The voltage will be high when the engine is cold and low when the engine is hot. By measuring the voltage, the control module knows the engine coolant temperature. The engine coolant temperature affects most other systems controlled by the module.