A heater blower motor resistor is an electronic device with three or more progressive resistors that allow the Heater Blower Motor to operate at slower speeds.
The highest speed of the Heater Blower Motor is not controlled by the resistor, so when the resistor fails, the fan will often work on the highest speed setting, but not the lower speeds.
To resistor is generally located under the dashboard, near the blower motor. The resistor's electrical connector is disconnected, the resistor is removed and the new resistor is installed. On some vehicles it may be easily accessible, and on others it may take partial dashboard removal to replace the resistor.