This post overlaps with a fuller one: Outside AC Unit Not Running? What the Sound Tells You.
When the outside unit stays silent in a Pasadena, CA summer and the thermostat is calling for cooling, the cause is nearly always power, a control, or one of two small parts that give up in the heat.
Start with what is running
Set the thermostat to Cool and drop the setpoint a few degrees, then listen at the furnace or air handler and at the outdoor unit. Nothing running at either end points at the thermostat or a safety switch indoors. Indoor blower running with a dead outdoor unit points at the condenser's power, its contactor, or the thin control wire.
The checks you can do safely
- Thermostat. Cool mode, setpoint below room temperature, fresh batteries if the screen is dim.
- Breakers. Usually two, one indoor and one outdoor. Reset a tripped one once. If it trips again, leave it off.
- The outdoor disconnect. The gray box on the wall by the condenser. The pull-out block gets knocked loose by gardeners and kids.
- The indoor service switch and blower door. A door left unlatched after a filter change stops the whole system.
- The condensate float switch. A clogged drain cuts the cooling call. Water in the pan is the tell.
- Filter, ice and clearance. Change a loaded filter, let any frost on the copper lines melt, and pull leaves away from the unit.
- The delay. Most systems hold the compressor off for a few minutes after a shutdown. Wait before deciding it is broken.
What the sound means, and what needs a tech
Silence usually means no power, a dead contactor or a blown low-voltage fuse. A hum with the fan not turning points at the run capacitor, the part that fails most often in a heat wave. Start-then-stop points at a pressure switch, often from a dirty coil or a low charge. Capacitors, contactors, fan motors, refrigerant and the compressor all belong to a licensed tech. An AC repair on a no-start is usually one part, and a yearly tune-up catches a weak capacitor before August does.
The full guide: Outside AC Unit Not Running? What the Sound Tells You.
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Written with AI assistance by Boost Services, Heating and Cooling team, and reviewed by our licensed technicians before publishing. Boost Services Heating and Cooling, C-20 #1119892.