AC won't turn on, or the outdoor unit isn't running

When the AC won't start, the cause is often power or a setting: thermostat batteries, a tripped breaker, the outdoor disconnect, or a float switch that shut things off. A few checks at the thermostat, the breaker panel and the outdoor unit narrow it down.

Also searched as: ac not turning on, ac won't turn on, ac unit outside not running, ac unit outside fan not spinning.

Before you start. If a breaker trips again right after you reset it, leave it off and call an HVAC technician.

An outdoor air conditioning condenser unit on a pad beside a house with shrubs trimmed clear

Every question and answer on this page, in order

Question 1: Is the thermostat screen lit?

Yes, it shows a temperature: go to question 2
No, the screen is blank: Likely cause: the thermostat has no power

Question 2: What mode and temperature is it set to?

Lennox suggests setting it at least 3 to 5 degrees below room temperature so it clearly calls for cooling.

Cool, at least 3 to 5 degrees below room temperature: go to question 3
Off, Heat, or set at or above room temperature: Likely cause: the thermostat isn't calling for cooling

Question 3: At the breaker panel, is an AC or furnace breaker tripped?

A tripped breaker sits in the middle or off position. The outdoor unit usually has its own breaker, and the furnace or air handler has another.

Yes, one is tripped: go to question 4
No, they're all on: go to question 5

Question 4: Reset it once, firmly off then on. Did it hold?

Open only the hinged panel door, never the metal cover behind it. Reset it only once. Energy.gov suggests letting the unit cool for five minutes before you reset it.

It held and the AC started: Likely cause: a one-time breaker trip
It tripped again, right away or soon after: A breaker that keeps tripping: call an HVAC technician

Question 5: Is there water in the drain pan, or around the furnace or indoor unit?

Many systems have a float switch that turns the AC off when the drain pan fills.

Yes, there's standing water: Likely cause: a float switch shut the AC off
No, it's dry: go to question 6

Question 6: Go to the outdoor unit with the thermostat on Cool. What do you find?

The disconnect is a small box on the wall near the unit. Trane notes some units also have a small cord that can come unplugged.

The disconnect box was off, or a small cord was unplugged: Likely cause: the outdoor unit's power was off
It hums or clicks, but the fan doesn't spin: Likely cause: a capacitor or fan motor
Power is on, but it is completely silent: Likely cause: a capacitor or fan motor

Possible results

Need the words for the phone call? Build a what-to-tell-the-technician checklist, or read when to call an hvac pro.