A malfunctioning outside AC unit is frustrating, especially during hot weather when you rely on your home cooling system the most. An outside air conditioning unit may not turn on for several reasons, whether it’s a simple issue, such as the emergency switch being off, or a more complex problem, such as a refrigerant leak—which only a licensed professional can repair.

Regular HVAC maintenance is important to ensure efficient operation and a long service life. Read our guide to learn more about what causes an outside AC unit to not turn on and how to hire a professional HVAC technician.

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Reasons Why Your Outside AC Unit Isn’t Turning On

A functioning AC unit can feel like it’s essential during the hot summer months. If your outside AC unit isn’t turning on, you’ll want to figure out why and get it working as soon as possible. Here are some possible reasons why your outside AC unit isn’t turning on and some troubleshooting tips.

Broken Thermostat

The thermostat controls your home’s temperature. If it’s broken, it can’t tell your system when and how long to run. Thermostat replacement is not a do-it-yourself (DIY) job, but a certified HVAC technician can properly install and calibrate the thermostat.

The Thermostat Is Not Set To Cool

Check the thermostat to make sure it’s set to “cool.” Someone else in the house may have raised the temperature or turned it off. Set the temperature lower than the current house temperature and wait to see if the room cools down. If not, it could be a different problem.

The Inside Switch Is Off

Outside AC units typically have a power switch near the attic furnace, closet, or crawl space. This switch looks like a standard light switch, and it’s easy for homeowners to turn it off accidentally. Flipping this switch off turns off the power to the blower, which will power down the outside AC unit.

The Shut-Off or Emergency Switch Is Off

Another switch that could cause your outside AC unit not to turn on is the emergency or shut-off switch next to the unit. This switch allows the cooling system to be turned off from the exterior.

Check to see if this switch is in the “on” position. You can also press the reset button on the outside AC unit or use the indoor system to reset the cooling system. After resetting the system, wait and see if the house cools down.

There’s a Blown Fuse or Tripped Circuit Breaker

A power surge or an overworked AC unit can trip the circuit breaker or blow a fuse. Go to your home’s main electrical panel and look for a tripped circuit breaker. Turn off tripped breakers and turn them on again. If it trips again after turning it on, you may have an electrical problem that requires an electrician’s help.

Older homes may have a fuse box with a blown fuse that needs replacing. The outside AC unit may also have a shut-off box with a blown fuse inside. In this case, an HVAC contractor may need to replace the fuse.

The Air Filter Is Dirty

A dirty air filter causes your air conditioner to run at lower efficiency, which can increase your energy bill and keep your home warmer than it should be. A dirty filter can block airflow to your AC unit, putting more pressure on the system and causing it to overheat and shut down.

Shut the AC unit off and open the access panels to the filter. Remove the old filter and replace it with the correct size and type of air filter. Pay attention to the arrow direction on the sides of the filter. These indicate the direction that the air is flowing through the unit.

The Capacitor Is Malfunctioning

If you hear unusual noises from the outside AC unit and it isn’t turning on, there could be a problem with the capacitor. This happens when the AC condenser tries to spin without power from the capacitor. 

An AC unit has a run capacitor and a start capacitor, which wear down over time. To test the capacitor, try spinning the condenser fan. If you have a dual-run capacitor and it starts spinning, there may be debris in the fan motor, or it’s damaged. If it doesn’t spin, the capacitor has malfunctioned. 

Clogged Evaporator Coils

Evaporator coils on an outdoor air conditioning unit get dirty quickly. This makes it difficult for your equipment to transfer heat, so the unit must work harder to keep your home cool. Dirty coils are also prone to corrosion and lead to refrigerant leaks.

This is why it’s essential to have an HVAC contractor clean your AC and perform preventive maintenance every year.

There’s a Problem With the Condensate Drain Line

Your air conditioner produces condensation that drains out through a drain line. When there are high humidity levels, these lines clog more frequently. If this happens, the line trips the safety switch to shut down your AC unit.

The Unit Is Low on Refrigerant

Refrigerant is a chemical compound found in copper coils in your outside air conditioning. As warm air moves over the coils, the refrigerant absorbs the heat and cools it down. If there’s a leak in the coils, refrigerant leaks out and the AC won’t turn on. Other signs include ice buildup outside the unit, warm air coming from the vents, and unusual noises.

If you suspect a refrigerant leak, only a licensed professional can fix this.

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DIY vs. Professional Outside AC Unit Repair

You can fix some outside AC unit problems yourself, but others require an HVAC contractor’s help. Here are some simple things you can do yourself and when to call a professional.

DIY Outside AC Unit Repair

Most problems with your outside AC unit aren’t do-it-yourself (DIY) jobs, but there are simple things homeowners can tackle before calling a pro. Here are DIY jobs that most homeowners can do:

• Replacing the air filter
• Flipping tripped circuit breakers on the main electrical panel
• Replacing a blown fuse in the fuse box
• Checking the thermostat to make sure it’s set to cool
• Rinsing the coils on the outdoor central AC unit
• Flipping the power switch
• Hitting the system reset button

Professional Outside AC Unit Repair

Hiring an HVAC technician costs $50–$100 per hour, not including parts. Technicians charge $100–$250 for an initial service call. Licensed professionals have the equipment and skills necessary to diagnose and repair complex problems. They can provide a more thorough and accurate diagnosis of any issues and offer targeted solutions.

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How To Hire a Professional for Your Outside AC Unit

When your AC requires more than a simple DIY repair, you’ll need to hire an HVAC technician. Keep these tips in mind when you review candidates.

• Start your search for an HVAC professional by researching potential contractors in your area online and reading reviews and ratings from past customers. 
• Look for contractors with specific experience working on outside AC units, and ask for references from previous clients.
• Check that the contractor is licensed, certified, and insured and verify their credentials with the state licensing board.
• Ask for a detailed estimate and don’t hesitate to get several quotes.
• Ask about warranties and guarantees on workmanship.

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Our Conclusion

There are many reasons why an outside AC unit may not turn on. You won’t be able to fix every problem on your own and may need to hire an HVAC professional. We recommend requesting quotes from at least three HVAC contractors before choosing one.

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FAQ About An Outside AC Unit Not Turning On

Why is my outside AC unit not turning on?

If your outside AC unit isn’t turning on, troubleshoot the problem by checking the circuit breaker, ensuring the shut-off or emergency switch wasn’t flipped, and resetting the system. If that doesn’t work, call a professional HVAC company to diagnose and fix the problem.

Is there a reset button on my outside AC unit?

Some outside AC units have a small red reset button. You can learn more from your HVAC manufacturer if you can’t find the button.

How do I know if my outside AC fuse is blown?

If you suspect your outside AC fuse is blown, use a voltmeter or multimeter to test the component. If it doesn’t get a reading, the fuse is blown.

Can you replace just the outside AC unit?

It’s typically recommended to replace both the indoor and outdoor AC units at the same time. This is because a new outdoor unit may not be compatible with the old indoor unit. Replacing only one could also void the warranty.

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