I have it on the auto fan setting

It's probably a relay switch that is bad. This needs to be properly tested to make sure that is the problem.

If the unit is a heat pump there is a relay on the defrost control that is probably stuck. The relay only breaks one side of the connection to the fan motor. A short to ground will make it run at half speed and burn it up completely. If, however, it's running at full speed you have a stuck relay. That relay is usually soldered on the circuit board controlling the defrost cycle. You can bypass the defrost board by unplugging the fan from the circuit board and plugging it onto the side of the contactor that feeds that side of the circuit. You will see one wire from fan plugged onto the contactor on one side of it. The wire you remove from circuit board will plug onto the other side so that when the contactor closes calling for the compressor to run the fan will run. Once you do this, push in contactor with a small screwdriver and both the compressor and fan will run. Make sure to unplug the power from outside unit before you do any work on it. Once you move the one wire turn power back on to test it. If you have plugged it on the wrong side the fan will not run and you will have to plug it on the other side. Make sure you plug it on the outgoing side, the incoming side hooks to power from the house. DrTom4444