Why Your Outside AC Unit Isn’t Turning On While the Inside Is

It’s a hot day, and your air conditioning is running inside, but you notice the outside unit isn’t turning on. As the indoor air stays warm and the temperature rises, it starts to get pretty uncomfortable. It’s definitely frustrating, but don’t worry – there are a few common reasons why this happens.

At HVAC.com, we’ll walk you through some potential causes for when your outside AC unit isn’t turning on, but the inside seems to be working fine, plus some bonus tips to help you figure out what’s going on and get your system back to normal.

1. Thermostat Settings and Malfunctions

One of the first things to check when your outside AC unit isn’t turning on is the thermostat. While it may seem obvious, it’s worth verifying that the thermostat is set correctly. Make sure it’s in cooling mode and that the temperature is set low enough for the system to kick on.

Sometimes, thermostats can experience malfunctions or wiring issues, which may cause the system to behave unpredictably. If the thermostat is not reading the temperature correctly or is stuck in heating mode, it may prevent the outside unit from turning on, even though the inside system is still running.

Solution: Check the thermostat settings to ensure they’re accurate and try resetting it or changing the batteries. If it’s still not responding, it may need to be repaired or replaced.

2. Tripped Breaker or Power Issues

If your outside unit isn’t turning on, a tripped circuit breaker could be the culprit. The outside unit operates on its own circuit, separate from the inside system. If there’s a power surge, overload, or electrical malfunction, the breaker controlling the outside unit may trip, cutting off power to it. This is a safety feature designed to protect your equipment from electrical damage.

Solution: Head to your electrical panel and check if the breaker for the outside unit has tripped. If it has, reset the breaker and see if the outside unit starts working. If it trips again, you might have an underlying electrical issue that requires professional attention.

3. Worn Fan Motor

The fan motor in your outdoor unit is responsible for circulating air over the condenser coils. If the fan motor becomes worn out or malfunctions, it can cause the outdoor unit to stop working, even though the indoor unit is still running. A worn motor may be due to age, dirt buildup, or mechanical failure.

Solution: An HVAC technician can inspect the motor, check for any signs of damage or wear, and determine if it can be repaired or if a replacement is necessary. Regular maintenance, like cleaning the motor and checking for signs of wear, can help prevent these issues from occurring.

4. Capacitor Issues

The capacitor is a small but crucial component in your AC system that helps start the fan and compressor motors. If the capacitor fails, the outside unit may not turn on, even though the inside air handler seems to be working fine. A faulty capacitor can result from wear and tear over time or electrical issues.

Solution: If you suspect a capacitor issue, it's important to have it inspected and replaced by a professional. Capacitors can hold an electrical charge, even after the system is turned off, which makes it dangerous to handle without proper training. An HVAC technician can safely diagnose the issue, test the capacitor, and replace it if necessary to restore proper function to the outside unit.

5. Defective Contactor

The contactor and relay are electrical components that send power to the outside unit’s compressor and fan motor. Unlike a capacitor, which stores and releases energy to help start the motors, the contactor directly controls the flow of electricity to the system. If either of these parts is defective, the outdoor unit won’t receive power and won’t turn on, often due to wear and tear over time.

Solution: This is another situation where you’ll need the help of a professional. An HVAC technician can test the contactor and relay and replace them if needed.

6. Dirty Outdoor Unit

A dirty outdoor unit, including clogged condenser coils, can prevent your system from working properly. When the coils are covered in dirt, debris, or grime, they can’t release heat effectively, which forces the system to work harder and can cause it to overheat or shut down.

Solution: To fix this, turn off your system and carefully clean the coils using a hose or a soft brush to remove any buildup. If the coils are heavily soiled or you’re unsure how to clean them properly, it’s best to call a professional HVAC technician to ensure it’s done safely and effectively.

7. Compressor Failure

A compressor failure is one of the most serious issues with an AC system and can cause the outside unit to stop running. The compressor is responsible for circulating refrigerant throughout the system, and if it stops working, the entire cooling process can be disrupted, leaving the system unable to cool your home.

Solution: Unfortunately, a failed compressor requires professional repair or replacement. Compressor failure is often a serious issue and may warrant considering a full system replacement, depending on the unit’s age and condition.

8. Airflow Blockage

Blocked airflow can cause the outside AC unit to stop working properly, or at all. If the air can’t flow freely through the condenser coils due to debris, dirt, or objects blocking the vents, it can lead to poor heat exchange, causing the system to overheat or stop cooling effectively.

Solution: To fix this, make sure the area around the outdoor unit is clear of debris like leaves, branches, or dirt. You should also check for any obstructions around the fan or vents. If you’re not comfortable cleaning it yourself, or if the blockage is inside the unit, it’s a good idea to have a professional HVAC technician inspect and clean it.