The Engine Coolant Temperature (ECT) is a sensor used to detect the temperature level in the coolant. This ECT changes a voltage signal from the Engine Control Module (ECM), then forwards that signal to the ECM, which interprets it as the engine coolant temperature input.
The ECT sensor uses a thermistor, which reacts to the change in temperature. When the temperature rises the resistance falls and when the temperature falls the resistance increases.
The Engine Control Module (ECM) observes slow changes in the resistance of the ECT sensor signal circuit. If you see a rapid change in voltage that does not match the engine temperature or see a lack of change in the ECT signal, the diagnostic code P0116 OBDII is set.