February 06, 2026
You hear your indoor unit running. Air is moving through the vents. But something feels off. The air coming out isn't cold, and when you walk outside to check, your condenser unit is completely silent. No fan spinning. No humming. Nothing.
When your outside AC unit is not turning on, but the inside is running, your home isn't cooling down, no matter how long you wait. This disconnect between your indoor and outdoor units is more than an inconvenience. Depending on what's causing it, continuing to run your system in this state can actually make the problem worse and more expensive to fix.
With over 25 years of diagnosing air conditioning problems throughout North Georgia, our family-owned team has seen every reason an outdoor unit fails to start. Some causes are minor. Others signal serious problems that need immediate attention. Here's what's actually happening when your outside AC unit won't turn on, and what each scenario means for your system.
Why Your Indoor Unit Runs While the Outdoor Unit Stays Silent
Your air conditioning system has two main components that must work together to cool your home. The indoor unit pulls warm air from your house and passes it over cold refrigerant coils. The outdoor unit releases absorbed heat and sends cooled refrigerant back inside.
These two units have separate electrical connections and controls. Your indoor blower can run independently because it responds directly to thermostat commands on its own circuit. The outdoor unit has separate circuits and safety switches that must all function correctly for it to start.
When only your indoor unit runs, you're essentially just circulating warm air through your home. No heat transfer occurs, so no cooling occurs. Running your indoor unit continuously while the outdoor unit sits idle can cause problems with your evaporator coil, potentially leading to ice formation. The sooner you identify why your outdoor unit won't start, the better your chances are of avoiding a more costly air conditioning repair.
What's Actually Wrong When Your Outside Unit Won't Start
Based on thousands of service calls across Canton, Woodstock, Roswell, and Alpharetta, here are the real reasons outdoor units fail to turn on and what each one means for your system.
Power Supply Problems
The most straightforward reason your outdoor unit isn't running is that it's not receiving power. Your outdoor condenser connects to your electrical panel through a dedicated circuit breaker, and there's typically a disconnect switch mounted on your house near the unit.
Circuit breakers trip when they detect an electrical overload or short circuit. A single trip might be a fluke caused by a power surge, but repeated tripping indicates something in your AC system is drawing too much current. Repeatedly resetting a tripping breaker without addressing the underlying cause can damage your compressor or create a fire hazard.
The disconnect switch near your outdoor unit can also be the culprit. Someone may have switched it off during maintenance or yard work. Some disconnect boxes contain fuses that can blow, cutting power even when the switch appears on.
A technician performing AC diagnostics will test voltage at multiple points to determine exactly where power is being interrupted and why.
Capacitor Failure
The capacitor is one of the most common failure points in outdoor AC units, especially in hot climates like North Georgia. This component stores electrical energy and releases it in a burst to start your compressor and fan motors. Without a functioning capacitor, motors can't begin spinning.
When a capacitor fails, you might hear a humming or clicking sound from your outdoor unit as the motors try to start but fail. A bulging, leaking, or burned capacitor has failed and needs replacement.
Capacitors degrade over time, and heat accelerates this process. While replacement is relatively affordable, the component holds a dangerous electrical charge even when the system is off. Ignoring a failed capacitor and continuing to try starting your system can burn out your compressor motor, turning a minor repair into a major expense.
Contactor Malfunction
The contactor acts as a relay switch, controlling power to your compressor and condenser fan. When your thermostat calls for cooling, it sends a low-voltage signal to the contactor, which then closes and allows high-voltage power to reach the motors.
Over time, the electrical contacts inside the contactor can become pitted, burned, or corroded. When this happens, the contactor may not close completely, preventing power from reaching the compressor. You might hear a chattering or buzzing sound as the contactor tries to engage.
Like capacitors, contactors are relatively inexpensive components, but catching the problem early prevents damage to more expensive parts downstream.
Refrigerant Issues Triggering Safety Shutdown
Your air conditioning system includes safety switches designed to protect the compressor. The low-pressure switch monitors refrigerant levels and, if pressure drops below a safe threshold, cuts power to prevent the compressor from running without adequate lubrication.
Low refrigerant is always caused by a leak somewhere in your system. Signs that low refrigerant might be causing your outdoor unit to stay off include gradual cooling decline over previous weeks, ice formation on refrigerant lines, and hissing sounds near the outdoor unit.
An experienced AC technician can test system pressures, locate leaks, and restore proper refrigerant charge after repairs are complete.
Compressor Problems
The compressor is the most critical and expensive component in your outdoor unit. It pumps refrigerant through the entire system, and when it fails, cooling stops completely.
Compressors fail for several reasons: running with low refrigerant levels can cause overheating, electrical problems can damage motor windings, and age eventually takes its toll. Before complete failure, compressors often show warning signs, including hard starting, short cycling, unusual grinding noises, and tripping the circuit breaker.
Compressor replacement is expensive enough that it often makes more sense to replace the entire outdoor unit, especially if your system is more than 10-12 years old. A technician can help you evaluate whether compressor repair or system replacement makes more financial sense.
Condenser Fan Motor Failure
The fan mounted on top of your outdoor unit pulls air across the condenser coils to release heat from the refrigerant. Without this airflow, heat can't escape, pressure builds, and the high-pressure safety switch shuts everything down.
Fan motors can fail due to worn bearings, electrical problems, or overheating. Before complete failure, a struggling fan motor often makes grinding or squealing sounds. Sometimes the motor is fine, but debris is physically jamming the blade. A technician will inspect for obstructions and test the motor to determine whether repair or replacement is needed.
Thermostat Communication Breakdown
Your thermostat sends signals to both your indoor and outdoor units to coordinate the cooling cycle. If the signal to your outdoor unit is interrupted, it won't know to turn on even though your thermostat is calling for cooling.
This communication breakdown can occur due to damaged low-voltage wiring, a malfunctioning control board, or problems with the thermostat. Before assuming the worst, verify that your thermostat is set to "cool" mode and that the temperature is at least 5 degrees below the current room temperature. If the display is blank, try replacing the batteries.
Why Waiting Can Make Things Worse
When your outside AC unit stops working, the temptation is to wait and see if it starts working again. Unfortunately, most outdoor unit failures don't fix themselves, and delay can escalate repair costs.
If your capacitor has failed and you keep trying to start the system, each failed start attempt stresses the compressor motor. Multiple hard-start attempts can burn out a compressor that might otherwise have been saved with a simple capacitor replacement.
If a low refrigerant level triggered a safety shutdown, the leak is still present and worsening. By the time you call for service, you're paying for both a leak repair and a larger refrigerant recharge.
The bottom line: when your outside AC unit won't turn on, getting a professional diagnosis quickly usually saves money compared to waiting.
What to Check Before Calling for Service
While most outdoor unit failures require professional diagnosis, there are a few safe checks you can perform that might save a service call.
Check your thermostat settings first. Verify it's set to "cool" mode and the target temperature is below the current room temperature. If your thermostat has batteries, try replacing them.
Look at your electrical panel. Find the circuit breaker dedicated to your air conditioning system. If it's tripped, reset it once. If it trips again within minutes, stop resetting it and call for service.
Inspect the disconnect switch near your outdoor unit. Make sure it's in the "on" position.
If none of these steps get your outdoor unit running, it's time to call for professional AC repair service.
Preventing Future Outdoor Unit Failures
Most outdoor unit failures are preventable with proper maintenance. Capacitors, contactors, and motors all degrade gradually before failing completely. Regular maintenance allows technicians to identify components approaching failure and replace them proactively, often avoiding emergency breakdowns during the hottest days of summer.
Annual maintenance also catches refrigerant leaks early, before they cause safety switches to shut down your system. Technicians clean condenser coils to prevent overheating, verify electrical connections are tight, and confirm all safety devices are functioning correctly.
If you haven't had your system serviced in over a year, scheduling preventive AC maintenance is one of the best investments you can make in reliable cooling.
Frequently Asked Questions
Why is my AC inside unit running, but the outside unit is not turning on?
Your indoor and outdoor units have separate power sources and controls. The indoor unit responds directly to your thermostat, but if the outdoor unit has a power problem, a failed component, or a tripped safety switch, it won't respond. Common causes include tripped breakers, failed capacitors, contactor problems, and low refrigerant triggering a safety shutdown.
How do I reset my outside AC unit?
Turn off your thermostat, then locate the disconnect switch near your outdoor unit and switch it off. Wait five minutes, turn the disconnect back on, wait another minute, then turn your thermostat back to cool mode. If the unit still won't start, a component has likely failed and needs professional diagnosis.
Should I keep trying to start my AC if the outside unit won't turn on?
No. If your outdoor unit doesn't start after 1 or 2 attempts, repeated attempts can damage components. Each failed start attempt stresses the compressor motor, and if a capacitor has failed, you risk burning out an otherwise healthy compressor.
How long do outside AC units last?
With proper maintenance, outdoor condensing units typically last 15-20 years. However, individual components such as capacitors and contactors may need to be replaced every 5-10 years. Compressors usually last the life of the system when properly maintained.
