A blower motor resistor is an electrical component that controls the power to the blower motor fan in a vehicles HVAC system. It is responsible for controlling the blower motor speed by limiting the current that runs through the blower motor. The blower motor resistor is connected in series with the blower motor assembly and has several resistors with different resistances. Each fan speed selection is connected to one or more of the resistors in the blower resistor assembly, and the change in resistance controls the current through the blower motor, which in turn limits the speed that the blower motor spins at. When high speed is requested, the blower motor resistor is bypassed, and the blower motor ground circuit is connected directly to ground, allowing the maximum current through the blower motor and maximum speed to occur.

The blower motor resistor can be a mechanical component that switches the blower resistor resistance mechanically using a rotating lever or an electronic module that sends a varied signal to the blower motor. The blower motor resistor produces heat when in operation, and the resistors are located in the path of the blower motor airflow to provide cooling to resistors.

The most common symptom of a bad blower motor resistor is that the blower motor only works in the highest setting or that it wont work at all. Other symptoms include only one speed from the blower motor, various airflow settings not working, and no air coming from vents.