The intake air temperature (IAT) sensor is a thermistor which changes value based on the temperature of air entering the engine. A low temperature produces a high resistance (100,000 ohms at -40°C/-40°F), while a high temperature causes a low resistance (70 ohms at 130°C/266°F).
The PCM supplies a 5.0 volt signal to the sensor through a resistor in the PCM and measures the voltage. The voltage will be high when the incoming air is cold, and low when the air is hot. By measuring the voltage, the PCM calculates the incoming air temperature. The IAT sensor signal is used to adjust the spark timing according to the incoming air density.