This post overlaps with a fuller one: Outside AC Unit Not Running? What the Sound Tells You.
An air conditioner that will not start in a Pasadena, CA heat wave almost always comes down to four things: power, the thermostat, a blocked or dirty outdoor unit, or a start component that has failed.
Work out what is running first
Set the thermostat to Cool, drop the setpoint, then listen indoors and outdoors. If neither end runs, the call is not getting out of the thermostat or a safety switch indoors has opened. If the indoor blower runs and the outdoor unit is dead, the problem is at the condenser.
The four causes, in the order we find them
- Power. A tripped breaker, an outdoor disconnect that got pulled, or the indoor service switch flipped off. Reset a breaker once. If it trips again, leave it off and call.
- The thermostat. Wrong mode, a setpoint above room temperature, dead batteries, or a loose control wire at the base.
- A blocked or dirty outdoor unit. Leaves and cobwebs against the fins raise pressure until a safety opens. Cut the power at the disconnect box by the unit, clear a couple of feet around it, and rinse the fins gently with a hose. No pressure washer.
- A failed start component. A hum with the fan standing still is the classic weak run capacitor. A click and nothing is the contactor or the compressor.
What needs a licensed tech
Capacitors hold a charge after the power is off, so leave them alone. Contactors, condenser fan motors, low-voltage fuses, refrigerant and the compressor all need testing, not guessing. A drain clog that trips the float switch stops the cooling call too. An AC repair visit on a no-start is usually one part, and a dirty condenser often needs a proper coil cleaning rather than a rinse.
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.