Hyundai Ioniq (AE): Engine Control System / Engine Coolant Temperature Sensor (ECTS). Description and operation
Engine Coolant Temperature Sensor (ECTS) is located in the engine coolant passage of the cylinder head for detecting the engine coolant temperature. The ECTS uses a thermistor that changes resistance with the temperature.
The electrical resistance of the ECTS decreases as the temperature increases, and increases as the temperature decreases. The reference +5V is supplied to the ECTS via a resistor in the ECM. That is, the resistor in the ECM and the thermistor in the ECTS are connected in series. When the resistance value of the thermistor in the ECTS changes according to the engine coolant temperature, the output voltage also changes.
During cold engine operation, the ECM increases the fuel injection duration and controls the ignition timing using the information of engine coolant temperature to avoid engine stalling and improve drivability.

Inspection1.Turn the ignition switch OFF.2.Disconnect the IATS connector.3.Measure resistance between the IATS terminals 3 and 4.4.Check that the resistance is within the specification.
Specification
Temperature
Resistance (kΩ)
°C
°F
-40-4048.
Other information:
Inspection1.Turn the ignition switch OFF.2.Disconnect the mode control actuator connector.3.Verify that the mode control actuator operates to the defrost mode when connecting 12V to terminal 3 and grounding terminal 4.Verify that the mode control actuator operates to the vent mode when connected in reverse.
Inspection1.Disconnect the negative (-) battery terminal.2.Remove the crash pad lower panel.(Refer to Body - "Crash Pad Lower Panel")3.Remove the lower crash pad switch assembly (A) after disengaging the mounting clip.4.Remove the rheostat switch connector (A).