AC Won't Turn On: What to Check and When to Call

Central air that won't start is usually being stopped before it begins: the thermostat isn't calling for cooling, a breaker or switch has cut power to the indoor or outdoor unit, a float switch has shut the system down over water in the drain pan, or a built-in delay is still running. Carrier and Bryant both say a tripped breaker, a thermostat setting, or a clogged filter is the most common cause. A failed part in the outdoor unit, such as the capacitor or contactor Lennox names, is a technician's repair.

If the system runs but the air isn't cold, see AC not cooling; if there's ice on the indoor coil or copper lines, see AC frozen coil.

Check these in order:

1. Thermostat: batteries, COOL, a lower setpoint, fan on AUTO.
2. The delay: wait 5 minutes after any restart.
3. Breakers: the indoor unit's and the outdoor unit's.
4. Switches and water: service switch, blower door, outdoor disconnect, float switch.
5. A humming outdoor unit: turn the system off and call.

Most Likely Causes, by What You See

First notice what runs. Lennox says a working outdoor unit makes at least a low hum and its fan blades spin.

The Thermostat Screen Is Blank

1. Dead batteries. Trane's first check: a blank screen may need new batteries. Lennox says fresh ones are often the whole fix on a battery-powered model.
2. The system has lost power. A hardwired thermostat draws power from the HVAC equipment, usually through a C-wire, Lennox explains. Honeywell's checks for a blank display: the circuit breaker, the power switch at the heating and cooling system, a securely closed furnace door, and a connected C wire. The first three are under nothing runs at all; for the last, see our C-wire guide.
3. A tripped float switch. Lennox lists it too: water backed up from a clogged drain line trips the switch, which cuts system power to prevent water damage.
4. A blown fuse or failed transformer. Lennox says a power surge can take out either, and the transformer is built into the HVAC system. Both are technician repairs; our thermostat troubleshooting page covers the fuse.

The Indoor Fan Runs, but the Outdoor Unit Is Silent

Set the thermostat to OFF between checks: Lennox says running the indoor fan alone "adds heat without cooling."

1. Thermostat settings. Trane says COOL, fan on AUTO, and a set temperature below the indoor temperature. Lennox suggests 3 to 5 degrees below room temperature, then a few minutes' wait.
2. A built-in delay. Carrier's owner's manual says some units have built-in time delays to protect the equipment. Honeywell says a flashing Cool On on its RTH6500WF means compressor protection: wait 5 minutes. Bryant describes a "3-minute rule" that keeps a compressor from restarting too soon. After a power blip, a breaker reset, or a settings change, give it 5 minutes.
3. The outdoor unit's breaker. Carrier says the outdoor and indoor units have separate breakers, the outdoor one usually 220 or 240 volts, and Trane says to check both. See how to reset one.
4. The disconnect box by the outdoor unit. Carrier and Bryant say to check it. Rheem's check before calling for service: the disconnect's fuses are in good condition and the power is on. If it's off and nobody switched it off for service, turn it on with dry hands. A blown fuse in it, Bryant says, usually needs professional replacement.
5. A frozen coil. Trane says a clogged filter can freeze the evaporator coil and shut the AC off; Lennox says a refrigerant leak can do the same and trip a low-pressure switch that protects the compressor. If you see ice, Carrier says to shut the system off; see AC frozen coil. A dirty filter is yours to replace (filter guide).
6. A failed part outside. With a cooling call and power on, Lennox points to a failed capacitor or contactor or a worn fan motor; Trane adds loose wiring and the compressor motor. All are technician repairs.

If other 240-volt appliances quit too, Florida Power & Light says damaged utility service leads can be the cause; see half the house has no power.

The Outdoor Unit Hums but Won't Start

Carrier says a hum from the outdoor unit usually means a motor is trying to start without the torque its capacitor supplies, and "the AC unit humming but not starting" is among the signs its technicians see most. Lennox agrees.

1. Turn the system off at the thermostat. Carrier recommends not running an AC with a bad capacitor, which can permanently damage the compressor and fan motors. Bryant says to turn the system off for buzzing, grinding, or hissing and call a qualified HVAC professional.
2. Don't open the unit. Lennox says a capacitor can carry a residual charge that could seriously hurt you. Bryant says not to disassemble the outdoor unit, and Trane says only a trained technician should touch its wiring.
3. Tell the technician what you heard. Lennox notes humming has other possible causes, so the diagnosis is theirs.

Nothing Runs at All

When a thermostat loses power, Lennox says, the equipment "has no instructions to follow," so start indoors.

1. Settings, then the fan test. With COOL and a lower setpoint confirmed, set the fan to ON: Rheem says the indoor blower should start immediately. If it doesn't, check the power below; then set it back to AUTO.
2. The furnace or air handler breaker. Lennox says an air handler has its own dedicated circuit, and a trip cuts its power completely.
3. The service switch. Trane says a furnace's shutoff switch may look like a light switch next to it; Lennox says some air handlers have a disconnect or breaker on the unit. Make sure it's on. If you smell gas at the furnace, don't touch the switch: leave the house without switching anything on or off or using a phone inside, and call 911 and the gas utility once you're a safe distance away.
4. The blower door. If the furnace's blower door is off or ajar, Trane's fix is to close it "to restore power to the furnace and the gas valve." Turn the service switch off, seat the panel, then turn it on. Carrier's furnace instructions say never to tape or bypass the door switch. More in our furnace guide.
5. A tripped condensate float switch. Trane says a clogged drain line can shut the system down if it has a condensate safety switch; Lennox says the switch trips when water in the pan under the indoor coil rises too high. Look for water in the pan or around the indoor unit. Clearing the drain removes the trigger, Lennox says; our condensate line guide shows how.

How to Reset a Tripped AC Breaker

The ESFi notes a tripped breaker can rest close to ON, so look carefully. Open only the panel's hinged door; the CPSC says panel boxes are usually live even with the main breaker off, so the screwed-on cover behind the door stays on for a licensed electrician.

1. Dry hands, dry floor. The ESFi says never to handle electrical items when wet, and CenterPoint Energy says to cut power only if the area around the panel is completely dry.
2. Thermostat to OFF, Carrier's first reset step.
3. Breaker fully to OFF, then ON, as the ESFi describes. Once.
4. Thermostat to COOL, then wait 5 minutes.
5. If it trips again, leave it off and call. Carrier says an immediate re-trip signals a deeper electrical fault; our breaker page explains why a second trip means stop.

Trane is stricter: it says not to flip a tripped AC breaker at all but to call an HVAC professional, because resetting it without knowing why it tripped could damage the system. Carrier says to reset one only if you're comfortable. If water has reached the panel, CenterPoint says don't touch it; call the utility to disconnect power from outside.

What You Can Fix Yourself

• Thermostat batteries, mode, setpoint, fan, and schedule.
• One breaker reset, the service switch, the blower door, and a disconnect left off.
• A clogged condensate drain.
• A dirty filter: Trane says replace it and try again, and call if the AC still won't run.
• Leaves and clippings on the outdoor coil, removed with the power off, without bending the aluminum fins, per Rheem.

Everything else is the technician's. Rheem says "under no circumstances should the Homeowner attempt to install and/or service the equipment," and Goodman says homeowners shouldn't open any equipment.

When to Call a Pro

• A breaker trips again, or a disconnect fuse may be blown.
• The outdoor unit hums or buzzes but won't start (Carrier, Bryant).
• The outdoor unit stays silent with a cooling call and power on: Lennox and Trane name the capacitor, contactor, fan motor, loose connections, and compressor, and Lennox adds the control board.
• Ice or a suspected refrigerant leak. The EPA requires Section 608 certification for technicians who service equipment that could release refrigerant, prohibits venting it, and limits refrigerant sales for home air conditioners to certified technicians.
• Water keeps coming back in the drain pan (Lennox).
• A burning or electrical smell near the unit, which Lennox says needs a pro right away. Turn the system off at the thermostat meanwhile.

Tell the technician what you saw and heard, and have each unit's model and serial numbers ready, as Carrier's manual suggests. If the system is past 15 years old and the compressor has failed, Trane and Bryant suggest weighing replacement against repair.

Staying Safe While You Wait

The CDC's heat guidance:

• Fans only below 90°F indoors; above that, the CDC says, a fan can increase body temperature.
• Get to air conditioning. Find a place that has it by dialing 2-1-1, checking the National Center for Healthy Housing's Cooling Centers by State page, or calling your local health department or emergency management agency.
• Drink water through the day and check on others, especially people who live alone. The CDC names people 65 and older, infants and young children, pregnant women, and people with heart disease or asthma as higher risk.
• Heat stroke is a 911 call. A temperature of 103°F or higher, hot, red, dry, or damp skin, a fast, strong pulse, confusion, or passing out: call 911 right away, move the person somewhere cooler, cool them with cool cloths or a cool bath, and give them nothing to drink.
• Heat exhaustion: heavy sweating, cold, pale, clammy skin, nausea, dizziness, or fainting. Move to a cool place, loosen clothes, use cool, wet cloths, and sip water. Get medical help right away if you're throwing up, symptoms get worse, or they last longer than an hour.

Prevent It

ENERGY STAR recommends a contractor check-up of the cooling system each spring, including tightening electrical connections, checking the controls and starting cycle, and inspecting the condensate drain. Carrier says annual tune-ups catch a capacitor drifting out of range before it fails. Check the filter monthly (ENERGY STAR), keep plants at least two feet from the outdoor unit (Trane, Bryant), flush the drain line with a little white vinegar every few months (Lennox), and replace thermostat batteries yearly (Lennox). Our summer maintenance guide covers the rest of the season.

Frequently Asked Questions

Why won't my AC turn on when the thermostat is on?

Confirm COOL, a setpoint below room temperature, and the fan on AUTO, then wait 5 minutes for the compressor delay. Then check both breakers, the service switch, the blower door, the outdoor disconnect, and the drain pan.

Why is my outdoor AC unit not running when the inside fan is?

Usually its breaker, its disconnect, or the delay. If those check out, Lennox points to the capacitor or contactor, both technician repairs. Turn the system off meanwhile.

How do I reset my central air?

Carrier's reset: thermostat off, AC breaker off, wait at least 30 minutes, breaker on, thermostat to COOL. Do it once. A breaker that trips again stays off until a technician looks.

Can I replace an AC capacitor myself?

No. Carrier strongly recommends against it because of the stored high voltage, and Lennox says not to replace one yourself. Call a licensed HVAC technician.

The Bottom Line

Thermostat, a 5-minute wait, both breakers, the switches, the outdoor disconnect, and the drain pan. Reset a breaker once with dry hands. Humming, a silent outdoor unit with power on, ice, and refrigerant go to an HVAC technician; the outdoor unit stays closed. If the house is dangerously hot, get to air conditioning.

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