Repair guide

AC not turning on

Updated 30 Sep 2026 · AR Logic LLC · 2 sources

Usually not an emergency

An air conditioner that will not start at all is usually being told not to: the thermostat is off, on the wrong mode or has dead batteries, a breaker or the outdoor disconnect is off, or a full condensate pan has tripped the float switch that shuts the system down. If the thermostat is lit and set to cool below room temperature and the breaker is on, the fault is likely inside the unit — a capacitor or contactor — and needs a technician.

Is an AC that won’t turn on an emergency?

Usually not. It becomes urgent in extreme heat for anyone elderly, very young or unwell — move them somewhere cool. Shut the system off and call for service at once if you hear buzzing, smell burning, or the breaker trips again when reset.

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What to check, in order

  1. Thermostat: power, mode and setting

    The screen must be lit, the mode on Cool, and the set point below the room temperature. Replace the batteries if the screen is blank or dim [1].

  2. The breaker

    Find the AC’s breaker (often a double breaker labelled AC or condenser). If it has tripped, switch it fully OFF, wait about five minutes, then ON. If it trips again, leave it off and call a technician [1].

  3. The outdoor disconnect

    Next to the outdoor unit there is a switch or pull-out block. Make sure it is on — it is easy to leave off after mowing or service.

  4. The air filter

    A badly clogged filter can make the indoor coil freeze and the system shut down. Check it monthly in cooling season and change it at least every three months [2].

  5. The condensate float switch

    Many indoor units have a float switch in the drain pan or line that cuts power when water backs up. If the pan is full, the drain line is clogged; clearing it restores power.

  6. Silent outdoor unit, humming, or clicking

    If the thermostat calls for cooling and the outdoor unit hums or clicks but the fan does not spin, a capacitor or contactor has likely failed — a technician’s job [1].

Safety first

  • Switch off the breaker and the outdoor disconnect before touching the outdoor unit.
  • Capacitors hold a charge after the power is off — do not open the electrical panel of the unit.

When to call a pro

Call an HVAC technician if the thermostat, breaker, disconnect and filter all check out, if the breaker trips again, or if you hear buzzing or smell burning.

What to photograph

For the Triage Profile, photograph the thermostat screen, and the outdoor unit with its label and the disconnect box beside it. One clear photo in good light is enough; the Triage Profile says what it is, how urgent it is (emergency, today, this week or whenever), and which trade to call.

Questions people ask

The thermostat is on but nothing happens.

Then batteries are probably not the problem. A tripped breaker or a failed capacitor or contactor is more likely [1].

How often should the AC be serviced?

ENERGY STAR recommends a contractor check-up each spring for cooling and each fall for heating [2].

Sources

  1. Carrier — AC unit not turning on
  2. ENERGY STAR — A guide to energy-efficient heating and cooling

Not sure what you’re looking at?

Snap a photo of it. The Triage Profile says what it is, how urgent it is, and who to call.