Central AC Not Working At All
If your central air conditioner won’t go on automatically when the thermostat signals the need for cooling:
1Check the main electrical panel and any secondary circuit panels for a tripped breaker or blown fuse. If you find the problem there, reset the breaker by turning it off and then on or replace the fuse. A central air conditioner typically connects to a dedicated 240-volt circuit.If the breaker continues to trip, suspect a short in the system—in the compressor, capacitor, or the fan motor. Hire an electrician to track down and solve this problem.
2Make sure the thermostat is set to COOL and its temperature setting is at least 3 degrees below the ambient room temperature.
3Make sure the power is on. Check the switch in the furnace or air handler, and the outdoor condenser. Also make sure no one has shut off the compressor’s 240-volt disconnect, typically in a metal box mounted near the compressor.
4Remove the thermostat’s cover after turning off the power to the air conditioner. Remove the thermostat’s body from the base (usually by pulling straight out) and replace the batteries (if it has batteries). Make sure all wires securely attach to their terminals and that the cover won’t pinch them. Replace the cover and wait 3 or 4 minutes, and then try the system again.
5If that doesn’t do the job, open the thermostat and unscrew the wire from the Y terminal. Turn the power back on. Holding the wire by its insulation only, touch the bare end to the R terminal and hold it there for about two minutes. If the compressor kicks on, the thermostat is faulty; replace it as discussed in the article How to Install an Electronic Thermostat. If the compressor doesn’t go on when you hold the Y wire to the R terminal, turn the power back off and either call an air-conditioning technician or check the capacitor.
6Finally, check the compressor’s capacitor and wires. The capacitor in the compressor starts both the condenser and the fan. If the capacitor has failed, the A/C unit will not run. It’s very easy to test whether it works and it is cheap and easy to fix if it doesn’t.
How to Test the AC Compressor’s Capacitor
Before opening the electrical cover on the A/C unit, be sure to shut off all power to the unit and verify that it is off. Next, remove the cover and, using a digital multi meter set to “Capacitance,” put one lead on the “Common” terminal and the other lead on one of the other two terminals. The meter should show a number—not “OL” which indicates a short. A/C capacitors can be like two capacitors in one, with both sharing the common leg.
Here is a video that shows more precisely how to do this. Tip: Take a quick digital photo of the wires before disconnecting them so you know where to replace them.