An air charge temperature sensor, or air temp sensor, is an engine sensor with a thermistor (a temperature-based variable resistor) which gets plugged in to the engine intake. The electricity flowing to and from the sensor increases and decreases with changes in air temperature, which allows the onboard computer to adjust for temperature changes.
Types of Thermistors
The two types of thermistors are called negative temperature coefficient (NTC) and positive temperature coefficient (PTC). With an NTC thermistor, when the temperature increases, resistance decreases. Conversely, when temperature decreases, the resistance increases. A PTC thermistor works in the opposite way, where as temperature increases, so does resistance, and when temperature drops, so does resistance.