The outdoor portion of your AC unit is what circulates outdoor and indoor air, as well as what cools it and moves it into your home. If it fails to come on, you may hear a clicking from the indoor air handler and possibly even the hum of a fan movement, but there will be no cool air coming in nor will the outdoor unit begin to hum. 

1. Failed Circuit Connection

An air conditioner is usually wired into its own dedicated circuit in your breaker box. If the circuit trips, such as from a power surge, then the AC won't come on. You can test the circuit by pushing it completely to the off position, waiting 60 seconds, and then turning it back on. If this doesn't work and the issue is determined to be electrical, an AC repair professional will need to check the wiring. 

2. Tripped Shutoff Switch

Most AC units have a shutoff switch on the exterior unit, designed to allow an easy interruption to the electricity in the event of repairs or maintenance. There is also the AC shutoff switch indoors on the thermostat. Locate and make sure both switches are flipped to the on position. If the switches are on and the unit still fails to start, then further troubleshooting is needed.

3. Clogged Condensate Line

The condensate line is what routes any condensation or moisture buildup away from the AC unit so it can drain without issue. If the condensate line becomes clogged, such as from algae growth or debris, it triggers the system to shut down so that moisture doesn't collect inside and cause problems. The condensate line must be cleaned or replaced before the unit will come back on.

4. Broken Fan Assembly

The exterior fan assembly is what draws in air from outside and pushes it through the AC. Belt-driven fans may fail if the belt needs replaced. Lack of lubrication can also cause fans to stop working, as can debris caught in the fan assembly. Cleaning the fan, followed by lubrication and belt replacement, should fix the problem.

5. Motor Failure

The motor runs the AC fan system. Damages to the motor, such as poor lubrication, damaged bearings, or mechanical breakdown, will cause the entire outdoor unit to shut down. An AC repair technician must diagnose and repair the issue in the motor. In some cases, replacement may be necessary.

Contact an air conditioning systems repair service for more assistance when the outdoor unit refuses to come on.