Most cars equipped with a manual heating, ventilation and air-conditioning (HVAC) system use a blower motor resistor to control the speed of the blower motor fan. The blower motor resistor is used to reduce the fan speed in lower settings (1,2 and 3). Some minivans and SUVs equipped with a separate heating and air conditioning system for the rear passengers use a separate blower motor resistor to control the speed of the rear blower motor fan.
but doesn't work in some lower speeds. The highest speed setting may still work because in most cars in the highest fan speed settings the current bypasses the blower motor resistor (see the diagram below). In some cars, if the resistor is failed, the fan may stop working completely. Another reason a blower motor resistor may fail is when the blower motor has a mechanical problem or when a foreign object jams the blower motor fan blade and prevents it from spinning freely. This leads to increased current through the resistor and causes it to overheat and fail.
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