Why Is the Outside AC Unit Not Turning On, but the Inside Is? 9 Common Reasons

The outside AC unit helps release warm air from inside your home to the outdoors, allowing the indoor unit to disperse cool air to keep rooms comfortable even in the summer heat. But when the outside AC unit is not turning on but the inside is, there could be a host of problems making you sweaty and uncomfortable. Here are some top reasons why your outdoor AC unit isn’t running as it should.

1. The Outdoor Unit Is Off

Many cooling systems have a power switch on the inside unit and on the outside unit. If you’re just powering up the AC system after a long winter, don’t forget to turn on both the indoor and outdoor switches. The power switch for an outside AC unit is usually in a metal box close to the outdoor AC unit. If it’s not there, you may find it in an attic, crawlspace, or basement.

2. There’s a Power Disruption

AC units need the appropriate amount of energy to run. Your AC may not turn on after a power outage, a circuit breaker trips, or there’s a power surge.

Check the circuit breaker. You can tell if you have a tripped circuit breaker if the breaker switch is in the off position, is centered between the on and off positions, or is showing a red tab. If you have a fuse box, look for signs of a blown fuse. Resetting a circuit breaker is easy to do yourself. However, if you reset the breaker and the outside AC unit is still not turning on but the inside is, it’s time to call in an AC repair professional for help.

3. The Contactor Is Failing

Contactors are important parts of an HVAC system that either lower to allow power to flow through the system or rise to block power. If the contactor is failing, it may not go down to let power reach the outside fan motor. Because contactors are essential components of a working AC system, it’s best to have a local air conditioning repair company repair a failing or worn-out contactor rather than trying to DIY.

4. The Fan Motor Is Worn Out

Over time, the fan motor on the outside AC unit can wear out, causing the outdoor unit to stop working even when the inside AC unit is on. The fan motor can last 10 to 20 years with proper maintenance, like annual inspections and regular AC unit cleanings, but it will wear down faster without regular maintenance.

A pro will need to come replace the fan motor for you. However, if your AC unit is more than 10 or 15 years old, you may consider whether you should repair versus replace the AC.

5. The Outdoor Unit Is Dirty

Whether it’s dust buildup on the coils or debris on the AC fan axle, dirt can prevent an AC unit from working properly. Dusty coils can stop the AC from cooling the air, while debris inside the fan axle can prevent the blades from moving. Even a buildup of debris like twigs and leaves around the exterior of the unit can lead to the outside AC unit not turning on even if the inside is. 

You may be able to clean the AC unit yourself if it means raking up leaves around the unit or cleaning the condenser coils, but the best option is to schedule regular cleanings of the AC unit at least once per year by a professional.

6. The Capacitor Isn’t Working

Capacitors store energy and provide a quick burst of power to the motor when you turn the AC on. A pro can test for a bad capacitor with a multimeter. Because testing and replacing a capacitor involves working with electricity, don’t try to DIY this fix.

If the capacitor is bad, you’ll need a replacement. Replacing a capacitor costs $100 to $250 on average for parts and labor.

7. The Safety Switch Is Engaged

HVAC systems have a safety switch, sometimes called a float switch or a condensate overflow switch, to prevent water damage when the AC pan is full of water. The switch will shut off the system to prevent the drain pan from overflowing.

Before turning the safety switch off, be sure to check if the drain pan is filled with water, which can happen because of a drain line clog, an improperly installed AC unit, or a dirty AC filter. If the drain pan is full, call a pro to find the source of the problem and make repairs before disengaging the safety switch.

8. The Thermostat Needs Adjusted

If your outside AC unit is not turning on but the inside is, head to the thermostat to check the settings. The set temperature should be lower than the current room temperature. If it’s not, adjust the temp to be cooler or shut off the AC if you don’t need it. 

If you set the thermostat temperature lower and the room temperature doesn’t cool down or the outdoor unit still doesn’t turn on, it’s time to schedule an appointment with a local AC repair expert.

9. The Compressor Failed

An AC unit depends on an HVAC compressor to move refrigerant through the system and cool the air. If the outside unit isn’t running, it could be because of a compressor failure. Other signs of a bad compressor include an AC unit that doesn’t blow out cold air and unusual noises, such as rattling or a loud stuttering noise when the unit tries to turn on.

If the compressor has failed, it’s time to hire an HVAC pro to replace the entire cooling system. While you could replace just the outdoor unit, it needs to be compatible with the indoor unit to work efficiently and extend its life span. That’s why it may be best to buy an entirely new AC system. Replacing your HVAC system costs $5,000 to $12,000.