10 Reasons Why an Outside AC Unit Is Not Turning On

By Amanda Lutz Updated February 6, 2024

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Central air conditioners contain two units: an indoor air handler and an outdoor condenser. Both must be running to produce cool air. If the indoor portion of the system is running and the outdoor condenser unit isn’t, warm air will blow from your vents.

This article outlines 10 potential causes of a nonfunctional outdoor AC unit, how to address them, and when it’s a good idea to call a professional HVAC technician.


1. The Thermostat Is Off or Not Set Correctly

It’s possible that someone in your home turned the thermostat off, pushed the wrong button when operating it, or set the temperature too high. Fortunately, fixing this is a simple matter of turning the thermostat on or adjusting the temperature. Set the temperature to “cool,” not “fan,” and reduce the temperature by a few degrees to see if this turns on the condenser.


2. The Thermostat Is Broken

Replace the batteries or visually inspect the electrical connection to make sure the thermostat has power. If it still doesn’t work, you may need to call an HVAC technician or an electrician to repair or replace the thermostat. Since the thermostat is wired into your home’s electrical system, this is often a job for a professional.


3. The Emergency Outdoor Switch Was Flipped

The outdoor AC condenser unit has its own power source and emergency shutoff to power it down if necessary. Go outside and make sure the switch to this unit hasn’t been turned off. It’s typically located in a metal box on the side of a house rather than on the unit itself. After locating the switch, move its pull-out head back into place to complete the circuit and allow electricity to flow to the unit. If the system doesn’t start, press the reset button on the outdoor unit first and then on the indoor unit. Call a professional to repair your system if the pull-out switch is damaged and can’t fit back into place.


4. The Indoor Switch Is Off

If the inside shutoff switch has been flipped off, the air conditioning system won’t blow cool air. Check the central indoor unit, which is typically located in an attic, crawl space, or closet. This shutoff looks like a regular light switch, so it may have been turned off accidentally. Turn it back on, and if that doesn’t kick-start your AC unit, try resetting the indoor unit.


5. The Circuit Breaker Tripped or a Fuse Is Blown

Power surges can trip circuits and blow fuses. Check your home’s circuit panel, assess if the corresponding circuit has been tripped, and turn it off and back on. If the circuit breaker immediately trips again, this may indicate a more serious electrical problem. Alternatively, if your home has a fuse box, you may need to replace the appropriate fuse. The AC unit itself may also have its own fuse, though if this one is blown, an HVAC professional will need to replace it.


6. The Capacitor Needs to Be Replaced

Air conditioners require a lot of electricity, and capacitors compose the part of your AC system that ensures the motors have the power they need to run. The start capacitor gets the unit running, and the run capacitor keeps the fan motor turning at a steady pace. Because capacitors deal with strong jolts of power, they can burn out and need to be replaced every 20 years. A bad capacitor often produces a humming noise or burning smell as the outdoor unit tries and fails to start.If you have a voltmeter, you can use this to test your capacitor. However, capacitors are high-voltage devices that are very dangerous to handle. If you suspect a capacitor is the problem, call a licensed HVAC technician to repair or replace it.


7. The Condensate Drain Line Is Clogged

Too much moisture in your system will cause the emergency shutoff switch to flip, and one cause of excessive moisture is a clogged condensate drain line. This tube drains water that condenses on the coils away from the system and can cause a water backup if it clogs with algae or grime. Very high environmental humidity can also cause clogs. If the emergency switch keeps tripping, check the condensate drain line by turning the AC unit off and using a wet-dry vac to remove clogs from the line.


8. You Have a Dirty Air Filter

One of the easiest and most important AC maintenance tasks a homeowner can do is change the air filter as the manufacturer recommends. In general, homeowners should change their air filters every three months. A dirty air filter will reduce overall AC performance before it affects the functioning of the outdoor unit. Since this is a simple do-it-yourself (DIY) fix, it’s worth changing the air filter at the first sign of a problem.


9. The Freon or Refrigerant Is Low

If there’s not enough refrigerant in the system, your outdoor AC unit may shut off to protect itself. However, fixing this problem isn’t simply a matter of adding more refrigerant. Low refrigerant levels indicate a leak in the system, and this toxic coolant must be recaptured and the leak fixed before more refrigerant can be added. Only licensed technicians can dispense the necessary chemicals, so call an HVAC company if you suspect a refrigerant leak.


10. The Evaporator Coils Are Dirty

An old, clogged filter forces your cooling system to work harder than it should, which can cause the evaporator coils to freeze or get dirty. Turning off the system will allow the ice to melt, but you’ll need to call an HVAC maintenance specialist to clean the coils if they’re covered in dirt and contaminants. The good news is that cleaning the coils tends to be more cost-effective than buying a new compressor or other AC component.


DIY Repair for Your AC Unit

HVAC repair is a complicated job that’s best left to trained and licensed technicians, but you can try a few DIY solutions before calling a professional.

  1. Check that the thermostat is working and set to the correct temperature.
  2. Change the air filter if you haven’t done so in the last 90 days.
  3. Reset the air conditioner system using the switches on the indoor or outdoor units.
  4. Check the emergency shutoff switches.
  5. Clean the condenser drain line and drain pan.

When to Call an HVAC Technician

Even seasoned DIYers should consider hiring an HVAC professional if they can’t get their AC unit to turn on. Air conditioners are large, complicated systems that use high-voltage electricity and dangerous chemicals, and attempting to work on the units without professional help can be dangerous. Additionally, without proper training and experience, you risk causing additional problems to your system and increasing repair costs.Remember to never attempt to repair electrical issues or deal with leaking refrigerant yourself. Even if you don’t harm yourself, you could damage the unit and shorten its life span.


Regular Maintenance Can Avoid Issues

While keeping up with AC maintenance won’t stave off all problems forever, it will prevent you from buying a new air conditioner before it’s absolutely necessary. Preventive maintenance may also be necessary to maintain the terms of your air conditioner warranty and keep your energy bills low. Here are some tips to keep your HVAC system working at peak efficiency:


Our Recommendation

If your outdoor AC unit won’t turn on, check the components above before immediately assuming you need a new AC condenser. While there are only a few DIY fixes you can reasonably expect to make, a licensed HVAC professional may be able to repair the problem without a costly replacement. To reduce the chances of an HVAC malfunction, keep up with regular maintenance tasks throughout the year.

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Outside AC Unit Not Turning On FAQ

How do I reset my outside AC unit?

To reset your outdoor condenser unit after a tripped circuit breaker or other cause, first power down the system at the circuit breaker box. Look for the reset button on the outdoor unit itself; it’s usually small and red. Hold the button down for three to five seconds, release it, and then turn the power back on at the breaker box.

How do I know if I need to replace my outside AC unit?

Here are some signs your air conditioner is nearing the end of its life span and may need replacement:
• The system is more than 10 to 15 years old.
• The unit doesn’t turn on.
• The unit regularly overheats or uses excess electricity.
• There are signs of serious electrical damage, such as a faulty breaker or loose wire.
• You hear excessive noise from the unit, which may be a sign of motor failure.

How much does it cost to hire a professional HVAC technician?

Often, the first HVAC service call to inspect and diagnose a problem will cost $100 to $250. After that, labor usually costs between $100 and $250 an hour, plus replacement parts.*
*All cost data via Angi.

How much does it cost to replace a thermostat?

On average, installing a thermostat costs $182, but this project can cost anywhere from $113 to $264. Smart thermostats will cost more to replace than traditional models.

How often should you replace an outside AC unit?

Most outside AC units need to be replaced every 10 to 15 years. If you need to replace either the indoor or outdoor unit, consider replacing the whole system at one time.