The ECT sensor (Engine Coolant Temperature) is a thermistor that is located in the engine block. This sensor of general form consists of two cables. One cable is the 5 volt reference of the Power Train Control Module and the other cable is a ground that is supplied by the PCM. When the coolant temperature changes, the resistance of the sensor changes as well.
When the engine is hot, the resistance of the sensor is low, and when the engine is cold, the resistance is low. If the PCM (Powertrain Control Module) discovers that the signal voltage is low to the range that allows the sensor to operate normally, the fault code P0117 OBD2 is established.