Outside AC Unit Not Turning On But Inside Is?
If the Outside AC Unit Not Turning on But inside is on, the issue is often due to a tripped breaker, faulty contactor, blown capacitor, disconnected thermostat signal, or a failed compressor.
Sometimes, it’s something simple like a clogged drain triggering a safety switch.
In other cases, it’s an electrical or mechanical failure in the outdoor condenser unit.
The inside system may still circulate air, but no cooling occurs because the outside unit handles heat exchange.
Let’s break it all down in this complete guide.
Indoor Unit Runs But No Cooling — What’s Going On?
When the indoor unit is blowing but no cool air is coming out, that’s a major sign the condenser isn’t running.
Your system needs both indoor and outdoor units to work together to cool properly.
The indoor unit moves air and contains the evaporator coil.
The outdoor unit releases the heat your AC pulls from your home.
If the condenser doesn’t run, your house won’t cool down.
Check The Thermostat First
Always start with the thermostat.
Make sure it’s set to “Cool.”
Set the temperature several degrees below room temperature.
If the indoor unit starts but the condenser stays silent, the signal may not be reaching the outside unit.
Replace the batteries if it uses them.
Check wiring for loose or corroded connections.
Older thermostats may lose accuracy or fail completely.
Is The Breaker Tripped?
One of the most common reasons the outside AC unit won’t turn on is a tripped breaker.
Air conditioners require a lot of power.
If there’s a surge or short circuit, the breaker trips to protect the system.
Go to the main electrical panel.
Look for the breaker labeled “AC,” “condenser,” or “outside unit.”
If it’s in the middle or off position, flip it completely off, then back on.
If it trips again immediately, stop and call a technician.
Outdoor Disconnect Switch Might Be Off
Near your outdoor condenser, there should be a shutoff box mounted on the wall.
This is a safety switch often used by technicians.
Make sure it’s in the “on” position.
Pull out the handle or switch and check that power is flowing.
A blown fuse in this box can stop the outdoor unit from working.
Failed Capacitor — A Common Problem
The capacitor helps the condenser motor and compressor start up.
If it’s failed, the unit won’t start even if it has power.
You might hear a humming sound or see the fan blade twitch but not spin.
Capacitors wear out with heat and time.
A swollen or leaking capacitor is a clear sign of failure.
This part should only be replaced by a trained technician due to electrical shock risks.
Faulty Contactor in the Outside Unit
The contactor is like a relay switch.
It receives the thermostat signal and allows power to flow to the compressor and fan.
If it’s burned or stuck, the outside unit won’t start.
Insects, corrosion, or wear can prevent it from closing the circuit.
Visual inspection may reveal blackened or pitted surfaces.
A voltmeter is usually needed to test it properly.
Compressor Failure
The compressor is the heart of the outdoor unit.
If it fails, the entire condenser may stay off.
Signs of a failed compressor include:
- Humming with no startup
- Frequent breaker trips
- Warm air blowing inside
A compressor failure is a major issue.
It often leads to a full system replacement.
This is more likely in systems over 10 years old or poorly maintained.
Fan Motor Malfunction
If the compressor tries to run but the fan doesn’t spin, the unit may overheat and shut off.
Sometimes, the fan motor burns out or the bearings seize.
You might hear buzzing or notice that the motor is hot to the touch.
Never stick anything in the fan blades to try to move them.
That can be dangerous and cause injury or more damage.
A technician can test motor windings and replace the motor safely.
Low Refrigerant Pressure
While low refrigerant alone doesn’t stop the outdoor unit from running, many modern AC systems have a low-pressure safety switch.
If pressure drops too low due to a leak, the compressor won’t turn on.
This prevents damage but leaves the system inactive.
Leaks can occur in the evaporator coil, condenser coil, or line set.
Signs of low refrigerant include:
- Ice on the indoor coil
- Hissing noises
- Weak airflow
- Longer run times
If the unit is low on refrigerant, it must be located and repaired before adding more.
Clogged Drain Line Triggering Float Switch
Modern systems often include a float switch in the drain pan or line.
If water backs up, the switch cuts power to prevent overflow.
This can interrupt the outdoor unit even though the indoor fan keeps blowing.
Check for:
- Full drain pan
- Water around the air handler
- Gurgling sounds
Flushing the drain line with vinegar or compressed air can clear small clogs.
Persistent backups may require professional cleaning.
Wiring Issues Between Indoor and Outdoor Unit
The outdoor unit needs a control signal from the thermostat or control board.
This signal travels through low-voltage wires to the contactor.
If those wires are damaged, disconnected, or chewed by rodents, the outside unit won’t get the message to start.
Check for:
- Exposed wiring
- Loose terminals
- Signs of rodent activity
Only a technician should handle wire testing or repair to avoid hazards.
Control Board or Relay Failure
Some systems use control boards or relays to pass signals between components.
If the relay inside the air handler is defective, it might never send voltage to the outdoor unit.
Control boards can also fail due to surges or moisture damage.
This requires electrical testing to confirm.
If no 24v signal is reaching the condenser, the indoor board may be the culprit.
Dirty Condenser Coils Overheating the Unit
Condenser coils must release heat efficiently.
If they’re clogged with dirt, leaves, or debris, the system may overheat and shut off.
Over time, poor airflow can damage the compressor or cause short cycling.
Signs of dirty coils:
- Outside unit feels very hot
- Loud operation
- Longer run times
Use a soft brush or hose to gently clean the coils.
Avoid using pressure washers, which can bend the fins.
Defective High-Pressure Switch
If the refrigerant pressure climbs too high, the high-pressure switch will shut off the condenser to prevent damage.
Causes include:
- Dirty coils
- Overcharged refrigerant
- Faulty fan motor
This is a safety feature but may indicate deeper issues that need professional inspection.
Seasonal or Installation Issues
Sometimes, units are accidentally left off after service.
Check that the disconnect was reinserted properly.
Verify that breakers were turned back on after winter storage or repairs.
New installations may suffer from wiring mistakes or configuration problems.
Insect Nests or Debris in Contactor
Bugs like ants can crawl into the contactor and prevent it from closing.
Debris or corrosion can block movement.
Open the access panel carefully and look for blockages or damage.
Clean out gently with a soft brush if needed, but turn off power first.
Incorrect System Sizing or Mismatched Units
If the indoor and outdoor units were replaced at different times, they may not be fully compatible.
An oversized indoor coil or mismatched refrigerant type can confuse sensors or lead to control issues.
Always match units by brand and model when replacing.
FAQs For Outside AC Unit Not Turning on But inside is?
Why is my inside AC working but the outside unit is silent?
Usually, power isn’t reaching the condenser.
It could be a tripped breaker, failed capacitor, or faulty contactor.
Can a thermostat cause the outdoor AC unit not to turn on?
Yes.
If the thermostat isn’t sending the signal, the outside unit won’t know to run.
What happens if the outdoor AC fan doesn’t spin?
The condenser can overheat.
This can cause safety switches to shut it down.
Is it dangerous if my AC outside unit isn’t running?
It’s not immediately dangerous, but you’ll lose cooling.
Long-term, it can strain the system and raise bills.
What’s the easiest fix for this problem?
Check the breaker first.
Reset it if it’s tripped.
Also, make sure the disconnect is fully inserted.
Can I replace a capacitor myself?
It’s possible, but dangerous.
Capacitors hold voltage even when off.
Use extreme caution or call a professional.
Will low refrigerant stop my outside AC from running?
Yes, if your system has a low-pressure safety switch, it will shut down the compressor.
Can bugs really stop my condenser from running?
Yes.
Insects can block the contactor or damage internal components.
Should I keep running the inside AC if the outside isn’t working?
No.
It’ll just move warm air around and strain the blower motor.
Conclusion
When your Outside AC Unit Not Turning on But inside is running, you’re dealing with an incomplete cooling cycle.
This situation wastes energy and leaves you uncomfortable.
It could be something as simple as a tripped breaker or as complex as a failed compressor.
Start with easy checks: thermostat, breakers, disconnects.
Then consider electrical parts like capacitors and contactors.
Dirty coils, blocked drains, and wiring faults can all play a role.
If you’re unsure or uncomfortable testing components, call a licensed HVAC technician.
Regular maintenance can prevent many of these problems before they interrupt your comfort.
Use this guide as your step-by-step checklist to solve the issue safely and efficiently.
