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Outside AC Unit Not Turning On

Every part of your AC system works together to keep your home cool, especially during the hot summer months. One of the most crucial elements of your cooling system is the outside AC unit. There are many components within this unit, and when one fails, the rest of the system will fail with it.

There are many issues that can prevent your outside AC unit from turning on, ranging in their severity.

Obvious and easy-to-fix causes include the shut-off switch being turned off, incorrect settings on your thermostat, or a tripped breaker switch. More complex issues involve the inner components of your outside AC unit; the air filter, evaporator coil, capacitors, fan motor, and compressor all have the potential to malfunction, preventing the outside AC unit from turning on. We’ve explained each of these issues in more detail and what actions you should take to get your outside AC unit back up and running. 

What is the Outside AC Unit For?

The outside AC unit is an important element of your air conditioning system, containing key components involved in the refrigeration cycle. Another name for the outside unit is the ‘condensing unit’ due to the action it has on the AC’s refrigerant. 

Within the outside AC unit, the main components are a compressor, a condenser coil, and a fan; the system also involves a refrigerant that has the job of transferring heat to turn warm into cold air. When the AC system is running, the refrigerant absorbs the warm air from inside your home. The compressor unit uses high pressure to compress the refrigerant into a gas, then pumps this gas through the system. 

The refrigerant gas reaches the condenser coil, where it releases its heat, before condensing back into a liquid form. Meanwhile, the fan within the outside AC unit works to exhaust the heat that the refrigerant releases to the outside air. The system follows this cycle to channel warm air to the outside of your home and cool air back in.

Now you know its basic operation, we can look at some of the reasons why your AC unit may not be turning on. 

Why is My Outside AC Unit Not Turning On?

If your outside AC isn’t turning on, it could be as simple as a tripped switch, or a more severe issue with the unit’s components. Below, we have run through each of the issues that may prevent an AC unit from turning on. We’ve started with the more obvious reasons before delving into those that are more complex.

1. The Main Shut-Off Switch to the AC Unit is Off

Most AC systems have a manual shut-off switch, typically located in an attic, closet, or crawl space near the system. It’s possible that this switch is flipped to the ‘Off’ position if your AC unit is in good condition but isn’t turning on. Someone could have accidentally flipped it, for example, if you’ve recently had the system serviced.

What to Check: Locate the main shut-off switch for your outside AC unit. This will look like a regular light switch. Ensure the switch is set to the ‘On’ position. 

2. The Thermostat isn’t Set to ‘COOL’

Your outside AC unit won’t turn on if your thermostat isn’t set properly. The system must be set to ‘COOL’ for the AC system to get going. It may be an obvious one, but it’s an easy mistake to make, especially if you adjust the thermostat regularly. 

What to Check: Make sure your thermostat is definitely set to ‘COOL’. Turn the setting on the thermostat to five degrees below the current temperature. The system will power on if there are no other issues. Otherwise, move on to the next step.

3. The Switch Has Tripped at the Circuit Breaker Panel

As you’re likely already aware, all of your home electrics are connected to a circuit breaker box in your property. The circuit breaker’s job is to protect your home and electrical appliances, including your AC unit, from high-voltage power surges. 

The circuit breaker will immediately cut off the power supply to the connected electrical circuit when it detects a high-voltage current. This will ‘trip’ the relevant switch in the circuit breaker box. Aside from high voltages, circuit breakers can also trip when multiple electrical appliances are running simultaneously on one circuit; they trip in response to the increased electrical demand on the system. This is more likely to happen if one or more of your appliances are older.

What to Check: Locate your circuit breaker box, and look over the panel to see if any of the switches are tripped. If so, flip the tripped switch back to the ‘On’ position. If the switch trips again, you’re best contacting an electrician to assess your home electrics. You may need to replace the AC unit or another electrical appliance in your home.

4. The Air Filter in the Outside AC Unit is Dirty or Blocked

The air filter in your AC unit collects water from the air, which then accumulates in the drain line. If the air filter becomes dirty or clogged with debris, these water droplets start to freeze. This can then cause the condenser coil and drain line to freeze over, which in turn prevents the AC unit from starting. You should be changing the air filter in your AC unit every 2 to 3 months.

What to Check: Take a look at the air filter in your AC unit and assess whether it appears clogged or overly dirty. If so, this could be the reason why your AC unit isn’t turning on. 

5. The Outside AC Unit Has Clogged Evaporator Coils

The evaporator coil in your AC unit is another part that can get dirty and clogged very quickly. This gradually reduces the efficiency of your AC system, and eventually may lead to the failure of the system entirely.

What to Check: To check the evaporator coil, you’ll need to remove the panel from your AC unit and use a flashlight to see into the coil assembly. This is a bit of a fiddly process, and even then, it can be difficult to tell whether the coil is dirty. It’s therefore best to have an HVAC specialist carry out this assessment for you.

6. The Outside AC Unit’s ‘Start’ or ‘Run’ Capacitors Are Faulty

Within the outside AC unit, there are a few different types of power capacitors that control its operation. Two of the main capacitors are called the ‘Start’ and ‘Run’ capacitors. These components can eventually become faulty as the unit ages, causing the AC unit to malfunction.

What to Check: Assessing the capacitors in an outside AC unit requires some skill and technical know-how. For this reason, only a licensed HVAC specialist should undertake this assessment. You should never try to check the capacitors yourself; they carry high levels of charge that could give you a severe electric shock in an instant. 

7. There isn’t Enough Refrigerant in the Outside AC Unit

Your outside AC unit requires a sufficient amount of refrigerant to work properly. If there’s a leak in the system, the refrigerant pressure will drop too low, stopping the AC unit from running.

What to Check: If you can see the condenser fan spinning but the AC unit isn’t turning on, this is an indication that the refrigerant level is too low. You shouldn’t run your AC if you suspect this is the case. You’ll need to contact an HVAC professional as chemical refrigerants require specialist handling.

8. The Motor in the Outside AC Unit is Faulty

Another part that can become faulty over time is the motor in the outside AC unit. This part is responsible for powering the fan within the unit.

What to Check: If you can hear the unit trying to operate but the condenser fan isn’t turning, the motor is likely faulty. All you can do in this circumstance is cut the power to the AC and contact an HVAC specialist for an assessment; checking for a faulty motor involves a capacitor test that must be carried out by a professional. 

9. The AC Unit’s Thermostat is Malfunctioning

Sometimes it’s the case that the thermostat is malfunctioning and failing to operate the AC unit properly. Your outside AC unit not turning on is one of several signs that the thermostat has gone bad. Thermostats typically last between 7 and 10 years; if yours is older than this, it may be time for a replacement. 

What to Check: Some signs of a malfunctioning thermostat include the thermostat showing incorrect temperature readings, not responding to your commands, short cycling, or forgetting programmed settings. Diagnosing this problem yourself can be a challenge, so again, it’s best to contact an HVAC specialist for this task. 

10. The Outside AC Unit Has a Blown Compressor

Leaving the worst for last, it’s possible that the compressor in your outside AC unit has broken down completely. This is also known as a ‘blown’ compressor. If the compressor blows, your AC system will no longer be able to supply the pressure needed to function correctly. 

What to Check: If you suspect your compressor has blown, you’ll need to contact an HVAC specialist for a diagnosis. This issue typically requires the total replacement of your outside AC unit. 

What to Do if Your Outside AC Unit Still Isn’t Turning On

After carrying out your own assessment, you may find that your outside AC unit still isn’t turning on. It’s always best to contact your local HVAC professionals to carry out an assessment when things go wrong with your AC system. 

From our guidance above, you can see that there are many potential reasons for a malfunctioning AC unit. Many of these issues are complex and require someone with specialist knowledge to diagnose and repair them. This is why we always recommend making your local HVAC professionals your first point of call, like us at Metro Heating and Cooling; our team of experienced technicians are always on hand to help out with any concerns you may have over your AC system. 


Schedule a HVAC repair anywhere in the Minneapolis or St. Paul area today by calling (651) 294-7798 or by requesting an estimate online.


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