AC Not Turning On: Which Half Is Dead and Why

By Ray Bennett · Updated 2 October 2026 · 4 min read · Checked against the sources at the end

AC Not Turning On: Which Half Is Dead and Why

Short answer

First work out which half is dead. Nothing at all, inside or out, is the thermostat, the switch on the furnace or air handler, a breaker, the blower door or the small fuse on the indoor board. Air inside with the outside unit silent is the float switch, the disconnect or the capacitor. The outside unit running with no air inside is the indoor blower or a frozen coil.

A central air conditioner that will not turn on can be dead in one of three ways, and each points somewhere different. Before touching anything, stand inside by a vent, then outside by the unit, and note which half is doing nothing.

This page sorts the fault by that one observation. Most of the causes cost nothing, because they are switches, breakers and settings rather than broken parts.

Before you start

What to carry: a torch and a pen. Twenty minutes.

Gas or electric? Electric. The indoor unit and the outside unit each have their own 240 volt or 120 volt supply, and a 24 volt signal from the thermostat ties them together.

Set it properly and wait. The thermostat on Cool, a few degrees below the room. Many thermostats then hold the compressor off for about five minutes.

What not to touch. The outside unit and the disconnect carry 240 volts. Check breakers and switches. Leave the inside of the units to a technician.

A breaker that trips twice. Reset a tripped breaker once. If it trips again, something is drawing too much current: why a breaker keeps tripping.

Never push the outdoor contactor in by hand. It forces the compressor on without its protections.

Which half is dead

One look inside and one outside splits the problem three ways, and each way has its own short list.

Check 1

Nothing at all, inside or out

The thermostat. A blank screen is batteries, or no 24 volt power from the indoor unit: when the thermostat is the problem.

The switch on the unit. Most furnaces and air handlers have a switch like a light switch on or near them. It is easily knocked off.

The blower door. The indoor unit’s panel presses a safety switch. If the panel is not pushed fully home after a filter change, nothing runs.

The indoor breaker. The furnace or air handler has its own breaker, separate from the outside unit’s.

The small fuse on the indoor board. A shorted thermostat wire blows it and everything stops: the air handler’s fuse and switches.

A full drain pan. On some systems the float switch cuts everything, on others only the outside unit. Look for water in the pan.

Air inside, the outside unit silent

Check 2Free to a service call

Follow it outside

The fan set to On. With the thermostat’s fan on On rather than Auto, the indoor blower runs all the time, cooling or not. Air at room temperature from the vents can then look like an AC that has stopped cooling. Check it is on Cool and below the room.

The call or the power is not reaching it. The float switch, the outdoor breaker and the disconnect on the wall are the first three: the AC outside unit not running.

A hum with no fan. Usually the capacitor: how an AC capacitor fails.

A blinking light on the unit. A diagnostic code: what the blinking yellow light means.

The outside unit running, no air inside

The four switches people forget. Each one can stop the system while everything else looks fine.

Check 3Free to a service call

Look at the indoor blower

The blower is not turning. The indoor fan motor, its capacitor, or the blower door switch: which fan has stopped and why.

Ice on the coil. A frozen indoor coil blocks the air. Switch cooling off and run the fan to thaw it: why an AC freezes up.

Air from some vents only. A duct has come apart, or the registers are shut: why no air comes from the vents.

After a power cut or a storm

Check 4Free, ten minutes

Give it a restart

Wait out the delay. Many thermostats and some outside units hold the compressor off for a few minutes after power returns.

Check the breakers again. A surge can trip the indoor or outdoor breaker.

Still nothing. The full list for heating and cooling after an outage is here: not working after a power outage.

Questions people ask

Why won’t my AC turn on at all?

If nothing runs inside or out, check the thermostat, the switch on the furnace or air handler, the indoor breaker, the blower door and the small fuse on the indoor board.

Why does my AC fan run but it does not cool?

The outside unit is not running. Check the float switch, the outdoor breaker and the disconnect, then listen for a hum that points at the capacitor.

How long should I wait after turning the AC on?

About five minutes. Many thermostats hold the compressor off that long after a change or a power cut.

Can a clogged drain stop my AC?

Yes. The float switch in the drain pan cuts the cooling when water backs up. Clear the drain and it usually starts again.

Where is the switch for my AC?

The indoor unit usually has a switch like a light switch on or near it, and the outside unit has a disconnect box on the wall beside it. Both have breakers in the main panel too.

Should I call someone if the breaker keeps tripping?

Yes. Reset it once. A second trip means a fault drawing too much current.

Why did my AC stop after I changed the filter?

The blower door or filter panel is not fully home, so its safety switch has cut the power. Push the panel firmly into place and it should start.

Why is my thermostat screen blank?

Flat batteries, or no 24 volt power from the indoor unit. That points at the switch on the unit, its breaker, the blower door or the small fuse on the board.

Sources: Resideo (Honeywell) T6 Pro installation guide 33-00181EFS (troubleshooting, fan modes, five minute compressor protection), Carrier 58SC0A furnace instructions (blower door switch, 3-amp fuse), Goodman *M9S80 installation instructions (blower door interlock), Trane "Troubleshooting the HVAC Air Handler Overflow Switch", Lennox "Outdoor AC unit not turning on" and Carrier "Will Frozen AC Fix Itself?"