A heater blower motor resistor is an electronic device that allows the heater blower motor to operate at slower speeds. Power is supplied from the fuse panel and it passes through the switch to the resistor. The resistor, then, slows down or speed up the blower motor as needed.
The heater blower motor resistor controls the lower speed settings of the motor. When the resistor fails, then the fan will only work on the highest setting and will need replacing.
Most manufacturer's factory warranty cover the heater blower motor resistor under the bumper-to-bumper warranty, if it fails due to a manufacturer defect or workmanship. There are extended auto warranties available that will extend coverage of the blower motor resistor after the factory warranty has expired. When purchasing an extended warranty, make sure to look at which coverage level includes the resistor.