The compressor fan is what keeps the air moving from the outside AC unit and into your home. Without it, there is no cool air coming in, so any problems need to be repaired promptly.

1. Debris Blockage

The most likely cause for an inoperative fan is debris that has made its way into the outdoor unit. Small twigs and branches, as well as blown-in leaves and garbage, can collect around the fan or become jammed in the fan blades. You may be able to fix this yourself by turning off the power to the unit to prevent the risk of shock and then removing the outer housing so you can access the fan. You can then pull out the debris so the fan can move again.

2. Dirt Buildup

A similar but less obvious issue is dirt buildup on the fan and fan shaft.  Pollen, dust, and other grime is the culprit. This dirt can also make its way into the bearing housing and gum them up so the fan either jams or simply doesn't rotate as well as it should. Your service tech can disassemble the fan to clean out the dirt and dust. They may also need to lubricate and repack the bearings. 

3. Failed Motor

The motor is what spins the fan, and it can fail for a variety of reasons. Sometimes the issue is electrical, and simply repairing the failed wiring will solve the problem. Motors, especially on older units, can also burn out completely at the end of their service life. Your tech can advise you on whether it makes more sense to replace the fan motor or if it is time to replace the entire AC unit. 

4. Capacitor Problems

A vital electrical component for the fan is the two capacitors, which are what store and release the energy that powers the fan. When a capacitor fails, the fan won't operate properly. If the problem is with the start capacitor, the fan won't begin to turn at all. Failure of the run capacitor means the fan may start up, but it will move slowly or quickly stop again right after startup. Fortunately, both capacitors can be replaced.

5. Belt Damage 

Many AC units still use belt-driven fans, which are hooked up to the motor via a belt rather than a gear. Over time, belts begin to wear out. They can stretch to the point that they are no longer functional, or they can break entirely. Inspecting the belt, and replacing it as necessary, should be part of your annual AC maintenance.

Contact an AC repair service if you are experiencing issues with your compressor fan.