The reason is that the passengers can feel cold inside the car if there is no heating system run. The motor works wonderfully in this case. In the case of heater motor, the heater resistor is a vital component of the heater motor. Therefore, when the motor stops to work, then you should check the resistor first. The reason is that the defective resistor will not hold the accurate current flow into the electronic circuit and make the heater motor non-functional. When you are buying the heater motor for your car, you should first check the resistor and make sure that will give you long service.