Short answer
Start at the thermostat: put in fresh batteries if the screen is blank, and set it to "Cool" a few degrees below room temperature. Reset a tripped AC breaker once. Make sure the disconnect box by the outdoor unit is on. Then wait five minutes, since many systems have a start delay. If it still does not start, call a technician.
Safety first
Touch the breaker panel only with dry hands, standing on a dry floor, and reset a tripped breaker only once. Never open the panels of the outdoor or indoor unit: parts inside can hold a charge even with the power off.
How to check an AC that won't turn on
1Nothing happens
No air from the vents and the outdoor unit is quiet. Most of the time the cause is a setting, a battery or a breaker.
Read all the steps
- Nothing happens. No air from the vents and the outdoor unit is quiet. Most of the time the cause is a setting, a battery or a breaker.
- Thermostat: batteries, then Cool. Blank screen? Put in fresh batteries. Set it to "Cool" and a few degrees below the room temperature.
- Reset the AC breaker once. In the panel, the AC breaker is usually a double one. If it sits in the middle, push it firmly to "Off", then to "On".
- Disconnect box by the outdoor unit: On. Someone may have switched it off while cleaning or mowing. Set it back to "On". Do not open the unit.
- Give it five minutes. Many systems wait a few minutes before they start, to protect the compressor. If nothing runs after that, call a technician.
- Still not fixed? Call an HVAC contractor Especially if: the breaker trips again after one reset.
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Other things that stop an AC
- The furnace or air handler switch. It looks like a light switch on or near the indoor unit and is easy to bump off.
- A float switch that shut the AC off because the condensate drain line backed up. Clear the line and it usually starts again.
- A furnace door that is not closed all the way. Many units have a safety switch that keeps them off until the door is in place.
- A thermostat schedule or a "Hold" setting that overrides what you set.
- A blown fuse in the disconnect box, a failed capacitor or a bad contactor. Those are jobs for a technician.
What not to do
- Do not reset a breaker that trips again right away. Leave it off and call.
- Do not open the outdoor unit or replace the capacitor yourself. It can hold a dangerous charge.
- Do not hold a breaker in the "On" position or tape it.
When to call an HVAC technician
- The breaker trips again after one reset.
- The indoor fan runs but the outdoor unit stays silent, or it hums without starting.
- The thermostat has power and is set to "Cool", but nothing runs after five minutes.
- You smell burning or see scorch marks on the panel, the disconnect box or the unit.
Frequently asked questions
Why won't my AC turn on?
The most common causes are dead thermostat batteries, a wrong setting, a tripped breaker or a disconnect box switched off. A float switch can also shut it off when the drain line clogs. You can check all of these yourself.
Is there a reset button on my AC?
Some outdoor units and furnaces have one, but many do not. The safe reset is the AC breaker: push it firmly to "Off" and then "On", once.
How long should I wait after resetting the AC breaker?
Give it about five minutes. Many systems have a built-in delay before the compressor starts.
Why is my thermostat on but the AC won't turn on?
Check the AC breaker, the disconnect box by the outdoor unit and the furnace or air handler switch. If those are on and nothing runs, a technician should check the capacitor, the contactor and the wiring.
Can a clogged drain line stop my AC from turning on?
Yes. A float switch shuts the AC off when water backs up in the drain. Clearing the drain line usually lets it start again.