The IAT sensor will usually be fitted somewhere in the inlet ducting after the air filter box or fitted on the inlet manifold.
The purpose of measuring the air temperature is to identify the density of the air. Cold air is denser than hot air and therefore more fuel has to be added to cold air than hot air in order to ensure optimal combustion. The density of the air is established by air temperature and air volume (as measured by the MAF sensor).
The IAT sensor is a variable resistor that changes with temperature. The higher the temperature, the lower the resistance and vice versa.
Using a 5V reference signal (sent by the ECU – Engine Control Unit) the resistance of the IAT sensor will decrease the signal in proportion to the colder air temperature. The resistance is a calibrated value, so as voltage drops across the IAT sensor the ECU establishes the air temperature from the voltage received.
IAT sensors consist of two wires, one wire for the 5V signal reference from the ECU and one for sending the decreased voltage back. Some IAT sensors are a combined unit with a MAF sensor and will contain 5+ wires.
To troubleshoot IAT Sensor error codes, unplug the IAT sensor and measure voltage across terminals from the ECU. You should see 5V otherwise the problem is elsewhere and not in the sensor.
Check for continuity on the IAT sensor terminals. If the IAT sensor is open (no continuity) it must be replaced.
The IAT sensor can be removed from the car and tested through its resistance range using an ohm meter and increasing the air temperature gradually using a hairdryer. You should see a gradual decrease of resistance as the air temperature increases. The actual resistance values depend on the particular sensor and the calibrated values saved in ECU, however expect 2-5 KOhms at normal air temperature (20 °C/70 ° F) decreasing to around 1 KOhms or less as the hairdryer heats up the air.