Negative Temperature Coefficient Sensors
Negative Temperature Coefficient (NTC) sensor's allow voltage to flow more freely when they're hot. The sensor in the illustration is an example of an NTC sensor. It's a thermistor, a type of variable resistor that changes resistance with temperature. Notice it has two wires that lead to the controller; one is its 5-volt reference signal and the other ground.
As resistance changes, the controller uses this signal to make output decisions based on seat temperature. If the seat is too hot, the controller slows or stops the flow of electricity through the elements. The module uses the signal from this sensor for safety and to regulate temperatures for HI and LOW settings.