Outside AC Unit Not Running But Inside Is

If your air conditioning (AC) system is blowing warm air when cool air should be flowing into your living space, there are several potential problems to consider. First, check whether the outdoor unit is working – the compressor is running and the fan is turning.

If the outdoor unit isn’t functioning at all or if it is buzzing to beat the band, we’ve got answers for you in this AC troubleshooting and repair guide.

What’s the problem if the outside ac unit not turning on? In most cases, a bad capacitor is the problem. It’s a cheap fix for an HVAC tech. And handy homeowners might tackle it too.

However, there are several other potential causes when the outside AC unit won’t turn on. One of them might more accurately explain your particular case. See our comprehensive list below, in order from most likely to least likely cause, to understand what is happening to your AC unit and whether or not the problem is DIY friendly.

Weakened or Failing Capacitor

The capacitor stores up the energy needed to power the outside AC unit fans. Therefore, if your capacitor is failing, you will hear a buzzing from your outside unit. This is because the condenser fan, located at the top of your unit, is trying to spin without the power to do so.

air conditioner new capacitor

There are two types of capacitors that can be found in an AC unit: run capacitors and start capacitors. A start capacitor provides the initial push the condenser fan needs to begin spinning, then disengages once the fan is in motion. Of the run capacitors, there are two subtypes:

  1. A single run capacitor alone starts the condenser fan motor and keeps it running. There is no start capacitor in a unit with a single run capacitor.
  2. A dual run capacitor powers both the condenser fan motor and the compressor. This run capacitor works alongside a start capacitor to initiate the condenser fan’s spinning.
Single and dual capacitor

A single run capacitor and a start capacitor have two terminals on top. A dual capacitor has three.

witing diagram AC capacitor

Note that the colors of each wire may differ depending on your unit, but the placement of each wire does not change.

No matter which run capacitor your unit has, each type is prone to fail over time. It eventually won’t be able to store enough juice to get the fan moving. They can also be easily impaired from power surges, overheating in extreme heat or from heating within the AC unit itself.

air conditioner capacitor bad and good

Some physical signs of a failing capacitor are when the capacitor is expanding on top, forming what looks like a mushroom head, or if the oil inside the capacitor is leaking out along its sides.

Helpful Tips

To test if the capacitor is malfunctioning, slide a thin yet sturdy wooden stick through the unit’s grate to gently push the fan into motion. Make sure to avoid using your fingers or conductive materials, such as metal, for this.

If the fan starts spinning on its own and your unit has a single run capacitor, then the capacitor is likely weak and on its way out.

If the fan does not spin at all, the capacitor has likely failed and there may be damage to the fan motor.

If the fan starts spinning on its own and your unit has a dual run capacitor, your unit is experiencing one of three possibilities:

  1. The start capacitor is failing and will need to be replaced.
  2. An abundance of debris or dust is jamming the fan axle or fan motor.
  3. The fan motor is damaged. This may be the only issue your unit is experiencing or an additional issue alongside the capacitor. For more on this, see the “Fan Motor Burn Out” section below.

If the fan does not spin and your unit has a dual run capacitor, then the run capacitor will need to be replaced.

How to fix the problem:

The capacitor will need to be replaced.

If the capacitor is not replaced in a timely manner, then the fan motor will have to work harder, which will lead to more issues. If your unit has a dual run capacitor, your compressor can be damaged, too.

How to prevent the problem later:

There is no surefire way to keep your run or start capacitors from failing. Yet, you can get a longer lifespan out of them by regularly cleaning the outside AC unit, so as to keep rodents from biting wires or making nests and to ensure debris does not damage the capacitor or the fan motor.

Is It DIY Friendly?

Changing the capacitor can be a DIY project. However, without HVAC expertise, it can be difficult to determine if the capacitor is the main problem, or if your unit has something else wrong with it. For this reason, we recommend contacting your nearest and most reliable HVAC technician to diagnose and fix the issue.


If you are planning on fixing the capacitor yourself, note that the price of the capacitor is dependent on the brand, how much charge it can store, and the size needed. The component can be found on Amazon.

In comparison, our AC Repair Guide shows that the average cost for a professional repair is anywhere between $125 and $375 for an hour of labor. This price may vary on the cost of living and whether the repair is made during “normal” business hours or after-hours as an “emergency” repair.


If you are certain a bad capacitor is the reason your outside AC unit not turning on and that your unit has no additional damage, refer to this basic procedure for a DIY repair:

  • Turn off the power before working on the outside unit. This means switching off the AC and furnace breakers, as well as pulling out the disconnect in the disconnect box nearby the outside unit. Close the disconnect box lid for safety.
  • Use a screwdriver or powered driver to remove screws to take off the access panel to the capacitor.
  • Use a voltage meter to see if the unit is fully powered off. Despite disconnecting the power, a unit can sometimes carry lingering charge.
  • Brush away any debris or dust around the capacitor and nearby electronic components.Note: If you have a dual run capacitor: Did pushing the fan start it up again? If it did, you will want to change the start capacitor only. If it did not, then you only change the run capacitor, which is typically grey/silver.
  • Do not touch the capacitor yet. To prevent shocking yourself, rub a tool like a screwdriver or pliers with plastic handles between the capacitor terminals to discharge any lingering voltage.
  • Loosen the bracket holding the capacitor in place.
  • Take the capacitor out.
  • Disconnect capacitor wires with pliers. Take note of where they are connected for later.
  • Replace old capacitor with your new one, plugging wires in their appropriate spots. Make sure that the new capacitor is the same type as the one you are taking out.

For more on this, refer to this video, which carefully and clearly moves through the proper steps to changing the capacitor.

Contactor Failure

When your external unit functions properly, a 24-volt current is sent to the compressor contactor. The current makes the contactor close and powers up the condenser fan motor in turn.

Compressor Contactor

While the contactor closes, there is an arcing, or curved trajectory, of electricity. This occurrence is normal, but it tends to heat up the contactor and can, over time, make its contacts stick together or weld shut in a closed position. With the contactor out of commission, the condenser will continue running, but the fan will stop spinning.

How to fix the problem:

Ultimately, the contactor will need to be replaced. But if your contactor is simply stuck and not welded in place, then you may be able to apply some temporary solutions while you wait to replace the device:

  1. Tap on the contactor to see if it will stop sticking. If the contactor begins working again, you may be able to get more time out of the component before replacing it.
  2. Use a brush to clean off the contactor. The dust or debris surrounding the contactor may be stopping it from working.

If either of these temporary solutions work, it is still in your best interest to have the contactor replaced sooner than later. This way, you can avoid any other damage to your AC unit that might cost you more money.

How to prevent the problem later:

The reason a contactor breaks down over time is because it is made out of copper, which is an inexpensive metal and a worse conductor than other, more costly metals. Therefore, the best way to prevent this issue in the future would be to purchase a commercial-grade contactor with gold, silver, or platinum contacts. These contactors are not cheap, but they are known for having longer lifespans. With one, you might never have to change the contactor again.

Otherwise, you should replace the contactor every three to five years as part of pre-emptive maintenance while you are cleaning the condensing unit or having it cleaned by an AC tech. This can ensure no expensive damage is done to your AC unit.

Is It DIY Friendly?

It is possible to make this a DIY project, but the repair is difficult and can lead to other AC unit damage or personal injury if you do not have a lot of experience. As with the capacitor, we recommend contacting an HVAC technician for the job.

According to our AC Repair Guide, the cost to have an HVAC professional replace a contactor is $125-$375. This price can vary depending on the price of the part and whether the repair is made during business hours or is an after-hours “emergency” repair.

Average AC contactor replacement cost is about $180 for a professional repair.

If you do plan on fixing the problem by yourself, then know that you can find a contactor—that is, a single-pole, 24 volt, 40 FLA amp. You can find this part here on Amazon.

For the experienced DIY homeowner, see this comprehensive video on how to replace the contactor. However, there are two things to note about this video:

  1. Instead of using your hands to pull the wires from the contactor, use needle-nose pliers. Wiggling the wires too much can cause damage to older wires and older AC units.
  2. Do not touch the capacitor before discharging it. To discharge, rub a plastic-handled tool between the capacitor terminals.

Condenser Fan Motor Burnout

Excessive stress on the fan motor is the underlying cause for burnout. However, there are a number of situations that can create additional stress on the motor. As previously mentioned, if your outside unit is running on a weak or broken capacitor, then the fan motor will be overstressed, eventually malfunctioning. Otherwise, natural unit wear and tear or a lack of unit maintenance will overwhelm the motor over time.

How to fix the problem:

The fan motor will need to be replaced.

One of the purposes for the condenser fan is to cool off the compressor. If a failing fan motor is not replaced within a reasonable time, then the compressor can overheat and break down. This would mean a higher repair cost for you, as the compressor can cost $2,000 and must be replaced by an HVAC professional.

How to prevent the problem later:

Having regular HVAC system inspections by an HVAC professional. We recommend having at least one yearly inspection for the best chance at avoiding this issue or catching it before too much damage can occur.

Is It DIY Friendly?

It is DIY friendly!

The condenser fan motor by itself can be found via Amazon, depending on the brand and the amount of voltage it can handle. Comparatively, a professional repair can cost upwards $225-$700 for two hours of labor. Make sure the motor is the same type as the one you are replacing.

Here is a basic outline of the replacement procedure:

  • Turn off the power before working on the outside unit. This means switching off the AC and furnace breakers, as well as pulling out the disconnect in the disconnect box nearby the outside unit. Close the disconnect box lid for safety.
  • Carefully remove the debris and sweep away the dirt surrounding the fan blades and motor.
  • Take off the grate shielding the condenser fan.
  • Remove the blades before removing the motor.
  • You will notice that the motor is wired to the unit. Carefully unplug the wires. Make note of where they were plugged for later.
  • Remove the motor.
  • With blades and motor removed, slide the new blades down the new motor rod to get it as close to the motor as possible.
  • Shorten the rod on the new motor with a handsaw.
  • Insert the motor and plug the new wires in the proper places.
  • Turn on the breakers and plug in the disconnect to see if the condenser motor will work.

For more on this, click here to see a professional replace three separate fan motors with three different wiring methods.

Compromised Power Supply

If your outside AC unit won’t turn on, there is a chance that it is not receiving the power it needs to run. Unfortunately, electrical issues can be difficult to diagnose, as there are a number of underlying reasons why your unit is not properly functioning.

Start by checking to make sure the connection in the AC disconnect box is secure. It can become loose if bumped. The box is outside near the condensing unit. Open the lid, and push the connections to ensure they are tight. If not loose, they might look “fried,” which can happen with lightning or another type of power surge. If so, an electrician or HVAC pro can repair it.

If your unit still does not turn on but you are certain that the capacitor, contactor, and the fan motor are healthy, then contact an HVAC professional to diagnose and resolve the problem.

2.A blown fuse or tripped circuit breaker. A tripped circuit breaker or a blown fuse means that your unit experienced a power surge at some point.

A breaker switches off by design to protect you from electrical fires. If your breaker tripped, flip the AC and/or furnace breaker back to “on.” If the unit begins running again, then keep a careful eye on it for a few days. If your AC unit operates irregularly after a few days, or if it does not turn on when you flip the breaker, then your unit is experiencing an electrical issue that an HVAC professional will need to address.

The average cost for this repair is $75-$325 for one hour’s worth of labor.

3. The thermostat is not set properly. Has it been turned to Heat or Fan? If so, the outside unit won’t turn on, and you’ll definitely get warm air out of the grates.

If the thermostat is properly set to Cool and the AC won’t come on, you might have thermostat trouble, if all the other issues check out OK. The thermostat wiring could be loose, or the entire thermostat might be bad. They rarely go bad, but it happens.

The average cost to replace a thermostat is $75-$325 for one hour’s worth of labor. Price depends on whether you install a basic programmable thermostat or upgrade to a nest or other smart thermostat. Our Thermostat Guide is a great place to research pros and cons of various thermostat types with links to brand guides for nest, ecobee, lyric and more.

DIY vs. HVAC Professional

Working with electrical components can be dangerous and lead to serious injury. Therefore, while we know DIY problem-solving can possibly help save you money and time, we recommend contacting an HVAC professional. Professionals can inspect your outdoor AC unit thoroughly to ensure it is experiencing no other damages or risks.

Work done by a professional usually comes with a warranty. For more information on the types of HVAC company warranties and what they cover, refer to this link.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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