A central air conditioner's exterior condenser unit contains a fan that keeps the condenser coils from overheating while changing refrigerant from gas to liquid. The fan plays an important role in maintaining the proper temperatures and preventing the entire unit from shutting down due to overheating. If your condenser fan has suddenly stopped running even when you turn on the thermostat, there are a couple of potential causes.
If you have some electrical or HVAC experience, you can check some of these problems out on your own. But it's better to leave the checks and fixes in the hands of John Legg's Heating & Air Conditioning or other capable heating and cooling repair technicians.
Faulty Compressor Capacitors
The cooling system starts at the indoor thermostat, which then sends a signal to the compressor in the condensing unit that it's now time to cool. The compressor fires up and pushes the gas refrigerant into the coils and then starts up the fan. If the compressor itself is faulty or broken, then the cooling process won't reach the step where the fan's participation is required.
Faulty compressors are often due to a broken start capacitor, which is an electrical device that provides an extra boost of electricity to help the compressor get started. You can test the capacitor using a multi-meter and your user's manual.
Turn off all power to the air conditioner before testing. You also need to discharge the capacitor of its stored energy by hooking up your multimeter, set to AC, and waiting for the number to drain to nothing. Switch the meter over the capacitor testing and make sure the final number matches what is written on the capacitor itself. If the number doesn't match, you need to buy and install a new start capacitor.
Overheated or Damaged Fan
The problem can also lie in the fan itself, as the fan can become overheated or otherwise damaged. If the compressor is running but the fan isn't, the fan is likely the problem.
You can check for general issues by turning off all power to the unit and then removing the condensing unit's cover. You should be able to locate the fan easily. Check the blades for severe bends or any larger debris that might have gotten inside the unit and stuck between the blades. Remove any debris but if there's other damage, you will likely need to replace the fan or call in an HVAC tech to replace the fan.