Typically, the heater fan resistor is located under a vehicle's dashboard in the heater and air conditioning compartment. It is part of a closed system that is generally not examined during routine maintenance inspections.
A vehicle's heater fan resistor enables its heating motor to change the speed of the air conditioner's fan. Without this part, the vehicle's heating system is only able to produce a high-pressure airflow or simply does not function at all.
Since routine maintenance technicians rarely check heater fan resistors during inspections, they must determine if the device is functioning poorly by verifying whether the air conditioning system works efficiently or not. This device may need to be repaired or replaced if the vehicle's air conditioning system is not changing smoothly between air speeds or is simply failing to push air through the ventilation system.
To verify a heater fan resistor, remove it from its compartment under the vehicle's dashboard and inspect the electrical circuits that lead into the device. If the electrical circuits are in functional condition, it is possible that the resistor itself is damaged or faulty. A qualified mechanic must replace it with a new or reconditioned device fit for the correct model of vehicle.