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1998 Ford Expedition

My heater/AC fan only works on high. how do i get the low and medium settings to work?

This is almost certainly a problem with the blower motor resistors.

The blower motor resistor pack is likely shot, and replacing it will likely solve the problem. If you install a new resistor pack and your fan speeds start working again, you should check your current draw on your blower motor. A dragging blower motor will cause the truck to eat resistor packs. If the motor is drawing too much current, it needs to be replaced.

thanks! I’ll look into this.