Outside AC Unit Not Running But Inside Is? 6 Culprits To Check

Central AC units and mini splits have two main parts – 1 outdoor and 1-8 indoor units – connected via the refrigerant and communication lines. When everything is working as it should, both indoor and outside AC units are running. If the outside AC unit is not running but the inside is, you do have a problem.

For example, you might see that the outside AC unit fan is not running but the inside is. In some cases, the reverse can be true as well; the outside AC unit is running but the inside is not. When this happens some homeowners even ask if the outside unit should run when AC is on.

Of course. Outside and inside units work in tandem. Both of them should be running at the same time to provide a cooling effect. If either of them has stopped (or didn’t even turn on) while the other is running, you will have to shut off the AC immediately and diagnose the problem.

Outside AC Unit Not Running But Inside Is
These are the worst problems that prevent outside AC unit from running. We cover all 6 culprits 1-by-1 further on.

Here is why one unit cannot work if the other doesn’t perform as it should:

  • Outdoor unit stores the all-important compressor. If the compressor is not working, the indoor evaporator coils won’t be cooled, and you will likely get warm air blowing from your AC unit.
  • Indoor unit stores the evaporator coils. They are cooled via the action of the compressor (outside unit). If the fan in the inside unit is not running, the evaporator coils will freeze over.

The bottom line is this:

If the outdoor unit is not doing its job, the indoor unit won’t be able to do its job properly as well (even though it’s running smoothly).

Here we will focus on the case of the outside AC unit not running but the inside is. Here is how you can approach this problem in general:

  1. Diagnoze why the outdoor unit is not running. If the outside AC fan is not spinning while the inside is, something is wrong. To fix your AC, you will have to adequately locate the problem. Below we list the 6 most common reasons why this happens. We will guide you step-by-step, eliminating the probable culprits and focusing on the one that prevents the outside AC unit from starting while the indoor unit is running smoothly.
  2. Fix the outdoor unit. For every one of these culprits, there is a fix. This ranges from something simple like flipping a switch or changing thermostat setting to serious fixed like a compressor or fan motor replacements.
  3. Troubleshoot the outdoor and indoor units.

Let’s look at two types of causes that prevent the outdoor unit from working properly while the inside unit is running as it would:

2 Types Of Problems (Wrong Settings Vs. Bad Parts)

Let’s say we notice our outside unit is not running while the inside unit is. When you break down what might be the cause of this, you can divide these issues into two categories:

  1. Wrong settings. You wish this is what you have; it’s very easy to fix. This includes outdoor on-off switch being accidentally switched, tripped breaker, or wrong thermostat setting. These problems are very easy to fix; we will go one by one and explain what is happening and how you can resolve the problem yourself.
  2. Bad parts. This is what you fear you have; fixing will usually need professional help and will cost quite a lot. This bigger fear is a broken compressor that needs to be replaced. Other parts that can go bad and can cause the outside unit to not run are bad fan motor (burnt out motor that spins the fan) or bad capacitors.

Let’s look at both of these types of culprits. We are going to start with the wrong settings causing the outside unit from running properly. Hopefully, that’s the problem in your case. We will continue with how to check if you have bad parts like the compressor or fan motor; this is not ideal but, you know, if you do have a bad part, you should just swallow that pill and call an HVAC guy to replace it:

#1 Check On-Off Switch

It may just happen that the outside unit is switched off. In the outdoor unit you have a small box that includes the on-off switch. Usually, you would switch off the outside unit during the winter when you don’t need air conditioning.

Now, your switch might be on ‘OFF’. This switch might be accidentally switched off and the result is that the outside unit is not running while the indoor unit is still running. Namely, by switching the outdoor unit off, you don’t actually switch the indoor unit off.

Solution: Just flip the switch to ‘ON’. That’s pretty much it. After you switch the flip, the outdoor AC fan should start spinning, the compressor will start running, and everything will be back to normal.

#2 Tripped Breaker In the Outdoor Unit Circuit

Breakers are electrical safety nets. If there is a surge of current that might damage the circuit or the AC itself, the breaker will trip, shutting off the circuit.

If that happens, you will see that when you want to run the AC, the outside unit won’t run but the indoor unit will run smoothly. Namely, the outdoor and indoor units each have their own breakers. We are usually talking about the standard residential 15 amp, 20 amp, or 30 amp breaker.

tripped breaker causing outdoor air conditioner failure
If your breaker keeps tripping, you should check the wiring and potentially increase breaker size to 30 amps.

What should you do if your outdoor AC unit isn’t running due to a tripped breaker?

Solution: Check if the breaker was just tripped or fried. If the outdoor AC breaker was just tripped, turn the breaker all the way off, and flip it back into the socket. This should do the job, your outdoor AC will again get the electricity it needs to run smoothly.

If the breaker was fried, replace the breaker. The easiest way to check if the breaker was fried is by flipping it back, starting the AC, and if you see that the outdoor AC doesn’t start running, you very likely have a fried breaker. Check which breaker you have and just buy a new one and install it (you can do that yourself, you can also opt to call an electrician to help you out).

#3 Check Thermostat Is Set To ‘Cool’

Again, this is a problem you wish you have. If your outdoor AC unit is not running properly but the indoor air blower is blowing warm air, the reason may simply be that you have the wrong thermostat setting.

Specifically, the thermostat is either:

  1. Not set to ‘Cool’. In this case, the AC won’t feel the need to cool your house. That means that the evaporator coils don’t need to be cooled and thus the outdoor compressor doesn’t really need to run. You will see that both outdoor and indoor fans are spinning, but you are not getting the cooling output.
  2. Thermostat is set above the current room temperature. Example: Indoor temperature is already 75°F but the thermostat is set to 76°F. This means that the AC won’t feel the need to provide cooling output. Therefore you will have airflow but that airflow won’t be cold, it will just be warm.
    setting thermostat too high can prevent outside ac unit from running while inside is
    Thermostat set temperature should always be under the current room temperature. This is the only way the outdoor unit will ‘feel’ the need to turn on the compressor.

In both of these cases, you can fix it very easily.

Solution: If the thermostat is not set to ‘Cool’, just switch it to cool. If, however, the temperature set on the thermostat is above the current room temperature, just scroll the temperature down.

In both of these cases, you will see that the compressor in the outdoor unit will magically ‘wake up’, the outdoor AC fan will start to spin faster and you will see that both indoor and outdoor units will start working in tandem as they should.

#4 Compressor Issues (Broken Or Misbehaving Compressor)

This is the worst fear when you see the outdoor unit not running while there is no issue with the indoor unit. The compressor – located in the outdoor unit – is the heart of every air conditioner, be it a central AC unit or a mini split.

If the compressor doesn’t adequately compress (hence the name) the refrigerant gas, you will see both the outdoor and indoor fans working but you don’t get cold air. You just get lukewarm or even warm air.

Solution: This is not a pleasant issue to solve. In most cases, when the compressor fails, you will have to replace the compressor. Compressor replacement is pretty much the most expensive AC service you can get. You will have to call a local HVAC specialist to replace the compressor, and the compressor can run for $1,000s. The replacement (labor) will also cost about the same as the compressor.

In this case, you, of course, have to first call your HVAC guy to confirm the reason your outdoor unit is not working as it should is indeed a failed compressor. If it is, you should start thinking about replacing the whole air conditioner, especially if you have an older (15+ years) unit.

In a lot of cases, replacing the air conditioner completely is a financially more viable option than replacing the compressor itself. If you need some contacts for local HVAC guys, we include a simple form at the end of the article: just fill the form, and you will get some quotes from HVAC experts in your area.

#5 Bad Fan Motor (Most Noticeable Culprit)

In most cases, we know that the outside AC is not running because the outdoor fan is not running. This is usually not because there is a problem with the fan itself; more often than not, the problem is with a bad fan motor.

A fan motor is exactly that; a motor that runs the fan. When the motor is broken or malfunctioning, it doesn’t power the fan and we see this as the outside AC not running.

bad fan motor stops outside ac from running smoothly
If the fan is not spinning, the culprit is usually a faulty fan motor.

How do you go about fixing the faulty fan motor preventing the outside AC fan from running?

Solution: Well, you will have to fix the motor itself, which is usually not an option. Once the AC fan motor fails, it’s notoriously hard to fix. That’s why in most cases, you have to replace the fan motor itself.

To fix a fan motor, you will have to call an HVAC expert. On your part, you can detect that the outdoor AC fan is not working, and call your guy. The replacing – which fan motor to use, how to wire it, and so on – is the domain of an HVAC expert.

Of course, replacing a faulty AC motor fan is not cheap. Nonetheless, it is much cheaper than replacing the compressor. That’s why even in older units, it makes sense to pay for the fan motor replacement.

Needless to say, once you have a functioning outdoor AC fan motor, the outdoor unit will run as it normally should.

#6 Bad Capacitors (Fan Motor Related)

When we see the outdoor AC fan not spinning while the indoor fan is spinning, the problem is most often the fan motor. If the fan motor itself is not broken, the most likely culprit are bad capacitors.

outside ac unit not running due to bad capacitors
Checking if there is still voltage coming out of the capacitors.

Capacitors are attached to the outdoor AC motor fan. They store and translate the electric energy that drives the fan motor which in turn runs the fan. Without working capacitors, you will see that the outdoor AC fan doesn’t spin but the inside fan is spinning.

Solution: Check if the capacitors are in contact with the fan motor. It might be that the contact is just not there; you have to push the capacitor back in contact with the fan motor.

If you see that that doesn’t help (turning the AC on and the fan in the outdoor unit is not spinning), you will have to replace the capacitors completely. In limited cases, you can do that yourself, or call an HVAC guy to do it for you. The key for DIY bad capacitor replacement is to pick the same capacitors as you had before; these are specific capacitors and there is no one-size-fits-all solution.

Who To Call If Outdoor AC Is Not Running While Indoor Is?

If you have an HVAC guy, you know who to call. If you don’t, you will have to find an HVAC expert in your area that can come and take a look at your outdoor unit.

Hopefully, you now know a bit more about why the outdoor AC unit is not running while the indoor unit seems alright.

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