Your central air conditioner is useless without a working fan. If the fan refuses to rotate and move air, the following are some of the most likely reasons for the problem.
1. Failed Motor
A failed motor means there will be no movement from the air conditioner at all. You won't hear it turn on and the fan won't even attempt to move. Often, there are loud whining or banging noises from the motor in the days leading up to complete failure, but not always. A technician will have to inspect the motor to determine if the problem can be repaired or if the motor will need to be replaced completely.
2. Damaged Control Board
The control board houses two important parts — the capacitor and the contactor. The capacitor stores up energy to power the fan, while the contactor determines when the fan turns on and off. If either of these parts fails, the fan won't operate at all even if the motor is running. Fortunately, both parts can be easily replaced.
3. Broken Belt
Some AC units are belt-driven instead of gear-driven, which means a belt connects the fan to the motor assembly. Eventually, belts wear out and either become too stretched to rotate the fan or they break completely. Belts should be inspected annually for signs of wear. Worn or broken belts can be replaced so that the fan works properly again.
4. Fan Assembly Failure
Several things can cause a failure in the fan itself. Fans have bearings in them that ensure smooth movement. If the bearings aren't properly lubricated, they will seize up and then the bearings must be replaced. Debris can also get trapped in the fan blades and wrap around the shaft, preventing it from moving. In severe cases, the fan blades may bend or break, which means the whole assembly must be replaced.
5. Loss of Power
A loss of power may occur if the wiring inside the AC becomes corroded or otherwise damaged. Rewiring is necessary in the event this occurs. Circuits can also become overloaded, leading to a loss of power. For this reason, an AC unit should be on its own circuit so it doesn't share power with other appliances. If you suspect electrical issues, always turn off the unit until a technician can look at it.
Contact an air conditioning contractor if your AC fan isn't running properly.