Why Would My Outside AC Unit Stop Working?

Last updated: October 2026

An outside AC unit usually stops working because of a tripped breaker or outdoor disconnect, a thermostat that isn’t calling for cooling, a failed capacitor, a bad contactor, a worn-out fan motor, an overheated or failing compressor, or debris blocking the fan. Lennox notes that even if the indoor unit turns on, the AC won’t work unless the outdoor condenser is working too.

What the unit is doing tells you a lot about the cause:

Outside unit still not running? AVS provides 24/7 AC repair across Northern Virginia and Maryland, with no extra charge for evenings, weekends or holidays. Call 703-457-9028 (VA) or 301-686-7129 (MD).

Quick checks you can do safely to see if your AC Unit Not Working

Start here before calling for service:

1. Thermostat. Confirm it’s set to “cool” and below the current room temperature, with working batteries.
2. Breakers. Check both the indoor and outdoor unit breakers in your electrical panel. Reset a tripped breaker once.
3. Outdoor disconnect. Check the disconnect box on the wall next to the outdoor unit.
4. Debris. Look for sticks, leaves or grass clippings around and on top of the unit.
5. Wait a few minutes. Many systems have a short startup delay after a power interruption or thermostat change.

Never open the outdoor unit’s electrical panel or reach into the fan area. The unit contains high-voltage parts and a fan that can start without warning.

1. Tripped breaker or outdoor disconnect

What you’ll notice: The outdoor unit is completely silent, while the indoor fan may still run.

Why it happens: The outdoor unit has its own high-voltage circuit. If that breaker trips, or the disconnect next to the unit is pulled or has a blown fuse, the condenser gets no power. Summers Plumbing Heating & Cooling lists a blown disconnect fuse among the causes when the indoor unit runs but the outdoor unit doesn’t.

What you can do: Reset the breaker once. If it trips again, leave it off and call a technician, since a breaker that keeps tripping points to an electrical fault.

2. Thermostat not calling for cooling

What you’ll notice: Nothing happens indoors or outdoors, or the indoor fan runs but the system never cools.

Why it happens: If the thermostat is on the wrong setting, has dead batteries or has a wiring fault, the outdoor unit never receives the signal to start. Some systems also hold the compressor off for a few minutes after a change.

What you can do: Check the settings and batteries, set the temperature a few degrees below room temperature, and give the system a few minutes. If you’ve recently installed a new thermostat, have its wiring checked.

3. Failed capacitor

What you’ll notice: A humming or buzzing sound from the outdoor unit, with the fan not spinning.

Why it happens: The capacitor gives the compressor and fan motor the burst of energy they need to start. Lennox explains that if you hear buzzing or humming from the outside unit, the capacitor may have failed and the condenser fan is trying to spin without power from the capacitor. Today’s Homeowner notes that capacitors fail from age, wear and electrical surges, and that a bulging, cracked or leaking capacitor needs immediate replacement.

What you can do: Turn the system off at the thermostat and call a technician. A failed capacitor is one of the most common outdoor unit repairs, but it holds an electrical charge and should only be handled by a professional.

AVS repair cost: $199 to $385 for capacitor replacement, according to AVS’s AC repair pricing guide.

4. Failed contactor

What you’ll notice: The outdoor unit stays silent even though the thermostat is calling for cooling, or it clicks or chatters without starting.

Why it happens: The contactor is the electrical switch that turns the outdoor unit on and off when the thermostat calls. Today’s Homeowner explains that if the contactor malfunctions and fails to close, it can’t send voltage to the compressor, so the system won’t start. Contacts can wear, pit or stick over time.

What you can do: Call a technician. Contactor work involves line voltage inside the unit.

AVS repair cost: $274 to $365 for contactor replacement.

5. Fan motor problems

What you’ll notice: The condenser fan won’t spin, spins slowly, or makes a grinding or squealing noise.

Why it happens: Lennox notes that a malfunctioning fan motor, often from worn bearings or general wear over time, can stop the fan and keep the outside unit from running. When the fan stops, the unit can’t release heat, and the system may overheat and shut down.

What you can do: Turn the system off and call for service. HomeGuide warns that a faulty condenser fan motor should be replaced right away, because the compressor may fail if it isn’t.

AVS repair cost: $585 to $1,264 for condenser fan motor replacement.

6. Compressor won’t start or keeps shutting off

What you’ll notice: The fan spins but the air inside isn’t cold, or the unit starts, runs briefly, then shuts down and won’t restart.

Why it happens: Lennox notes that if the fan blades are spinning but the air feels like it’s just coming from a fan, the compressor might have issues. Compressors also have a built-in thermal overload, as Alpha Mechanical explains, which shuts the compressor off if it overheats.

What you can do: Turn the system off and let it rest, then call a technician to find out why it overheated. HomeGuide notes that a hard start kit gives the compressor an extra boost on startup and may rescue a failing compressor, while a failed compressor is one of the most expensive AC repairs.

AVS repair costs: $100 to $250 for a hard start kit and $1,495 to $3,495 for compressor replacement.

7. Debris or a blocked fan

What you’ll notice: The fan turns slowly, rattles or won’t spin, and the unit may run hot.

Why it happens: Lennox points out that the outdoor unit can pick up dirt, sticks and leaves, which can get caught in its components and stop the fan from spinning correctly.

What you can do: Turn off power at the breaker and the disconnect, then clear loose debris from around the outside of the unit and off the top grille. Don’t reach inside the fan housing. If the fan still won’t turn freely, call a technician.

What you can check vs what needs a technician

How much do outside AC unit repairs cost?

Most outdoor unit repairs are single-part electrical fixes. AVS’s AC repair pricing guide lists these typical 2026 costs:

for the full list, see our AC repair cost guide. If your indoor unit is running but the house still isn’t cooling, the problem may not be the outdoor unit at all. Our guide to an AC running but not cooling the house covers the other common causes.

When to stop and call for AC repair

Turn the system off at the thermostat and call a technician if:

• The outdoor unit hums or buzzes without the fan spinning
• A breaker trips again after you reset it
• You smell burning or see scorch marks on the unit
• The capacitor on top of the unit looks bulged or is leaking
• The fan makes grinding or screeching noises
• The unit starts and shuts off repeatedly

If the AC has completely stopped and you’re waiting for help, here’s what to do when your AC breaks down, including how to keep your family cool in the meantime.

24/7 outside AC unit repair in Northern Virginia and Maryland

AVS provides 24/7 AC repair across Arlington, Fairfax, Loudoun and Prince William counties in Virginia and Montgomery and Prince George’s counties in Maryland, with same-day service and no extra charge for evenings, weekends or holidays. Site visits start at $90.

• Founded in 1991, with more than 50,000 residential and commercial customers served across the Washington DC metro area
• Licensed in Virginia (HVAC 2705039269A) and Maryland (HVAC 520714137895, Home Improvement 4192876)
• Call 703-457-9028 (VA) or 301-686-7129 (MD)

Frequently Asked Questions

Why does my outside AC unit keep tripping the breaker?

A breaker that trips repeatedly is warning you about an electrical problem. Today’s Homeowner explains that frequent breaker tripping signals a serious electrical issue, such as a short, an overloaded circuit or a failing capacitor. Don’t keep resetting it. Leave the breaker off and have a technician find the cause.

What should I hear when my outdoor AC unit starts?

A quick, predictable sequence. Alpha Mechanical explains that you should hear the contactor click within about 30 seconds of the thermostat calling for cooling, the fan spin within another 5 seconds, and the compressor settle into a steady hum within 10 seconds after that. A unit that hums for more than 30 seconds without the fan spinning needs service.

How long should an overheated AC compressor rest before I try again?

A few hours. Alpha Mechanical notes that when a compressor’s internal thermal overload trips, it can take two to four hours to cool down before it will restart. If it keeps tripping, something is making it overheat, such as a dirty condenser coil or a failing fan motor, so have it checked.

Can a power surge stop my outside AC unit from working?

Yes. Today’s Homeowner lists electrical surges among the reasons a capacitor can fail, and a failed capacitor can stop the outdoor unit from starting. If your AC stopped working right after a storm or power outage, check the breakers and disconnect first, then call a technician if the unit hums without starting.

Should I cover my outside AC unit in the winter?

Usually not with a full cover. Lennox explains that covering an AC unit completely, especially with plastic or vinyl, can trap moisture that leads to rust and corrosion, can even freeze the coils, and may void the warranty depending on the manufacturer. If your outdoor unit is a heat pump, never cover it, because it runs all winter.

Is it normal for my outside unit to run in the winter?

It depends on what you have. A cooling-only air conditioner sits idle in winter while a separate furnace heats the house, as the Farmers’ Almanac notes, so it shouldn’t run. A heat pump looks similar but heats your home, so its outdoor unit is supposed to run during the heating season.