Homeowner answer
Why won't my AC turn on?
When an AC will not turn on, the cause is often the thermostat, a tripped breaker, a clogged condensate drain that has set off the float switch, or a failed electrical part. Check the thermostat, the breakers and the drain safety switch first. If the system still will not start, book a diagnosis.
Call soon
Not dangerous in itself, but the house will heat up quickly in Florida weather. Book soon if the simple checks do not bring it back.
Updated
When nothing happens at all, the cause is usually something you can see from the outside: a setting, a switch, or a safety device doing its job. These checks take about ten minutes and need no tools.
What you can safely check
- Check the thermostat
Do this:The screen should be lit, the mode on Cool, the fan on Auto and the setpoint a few degrees below the room temperature. If it takes batteries, put in fresh ones.
You should see:Most systems start within a minute or two of the thermostat calling for cooling; some wait a few minutes on purpose to protect the compressor.
When to stop:Stop and move to the breakers if the screen stays blank after fresh batteries — and to the drain check after that.
- Find the indoor unit's switch
Do this:Look near the indoor unit — in the closet, the attic, the garage or the hallway — for a switch that looks exactly like a light switch. It switches power to the air handler.
You should see:The switch should be up, like a light that is on. Flip it up if it is down, then give the system a few minutes.
When to stop:Stop and leave the switch off if you smell burning or see scorch marks around it — that is one for a technician, with the power off.
- Check the breakers
Do this:At the electrical panel, find the breakers labeled for the AC, condenser, furnace or air handler. A tripped one sits in the middle or looks different from the rest. Switch it fully Off, then On, once.
You should see:With power back, a blank thermostat should light up and the system should start within a few minutes.
When to stop:Stop and leave the breaker off if it trips again. Repeated resets can damage the unit and the panel.
- Look for water at the drain
Do this:Look at the pan under the indoor unit and at the small switch on the drain line, if there is one. You are looking for standing water.
You should see:The pan should be dry or nearly empty, and the floor and unit around it should be dry.
When to stop:Stop and keep cooling off if there is water — the float switch has probably cut power to prevent an overflow, and the drain needs clearing by a pro.
- Check the filter door
Do this:If you changed the filter recently, make sure the filter door or cover at the indoor unit is fully closed and latched.
You should see:The cover should sit flush. Some units refuse to run with it ajar, because a switch inside detects the open door.
When to stop:Stop if the door will not latch — a filter in backwards or the wrong size is usually the reason. Reseat it or use the correct size.
Likely causes
- Thermostat settings or dead batteries
A thermostat set to Off or Heat, or one with dead batteries, never tells the system to start.
Handled by you
- Tripped breaker or switched-off service switch
No power means no start. A breaker that trips again after a reset points to a fault.
Handled by you
- Clogged condensate drain set off the float switch
The switch is doing its job by stopping the system before water overflows. The drain needs clearing, and the reason it clogged should be found.
Handled by a technician
- Blown low-voltage fuse or failed transformer
These supply control power to the thermostat and system. A blown fuse usually has a cause, such as a shorted wire, that also needs fixing.
Handled by a technician
- Failed control board, capacitor or contactor
When a part that starts the system fails, the thermostat can call for cooling and nothing happens.
Handled by a technician
Call a pro when…
Book a diagnosis if the thermostat has power and the right settings, the breakers are on and there is no standing water at the drain, but the system still does not start.
- Nothing runs after the thermostat, the switches and the breakers all check out.
- A breaker trips again after a single reset.
- There is water in the pan under the indoor unit or at the drain switch.
- The thermostat screen stays blank after fresh batteries and a reset breaker.
- You smell burning or see scorch marks near the indoor unit or the panel — keep the system off.
- The outdoor unit hums or clicks but never starts.
Next step
Book a diagnosis
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Common questions
Why won't my AC turn on after a power outage?
A breaker may have tripped when power returned, or a surge may have damaged a part. Check the breakers and thermostat, and allow a few minutes, because many systems wait before restarting to protect the compressor.
Can a clogged drain stop my AC from turning on?
Yes. Many systems have a float switch that shuts the AC off when condensate backs up, which protects your ceiling from a leak. The drain needs clearing before the system will run again.
Is it safe to keep resetting the AC breaker?
No. Reset a tripped breaker once. If it trips again, there is a fault that needs a technician, and repeated resets can make it worse.