All cars are designed with a heater fan motor that pushes air through the venting system. This air is processed by a series of vents to direct the air to various positions. For example, heater, air conditioner or defrost modes. Most heater motors are powered by a basic electrical circuit that utilizes a multi resistor or a solid state resistor to control different fan speeds (low, medium & high). If the resistor malfunctions then only circuit remaining is the high speed setting.