AC Running but Not Cooling? 9 Things to Check Before You Call

Quick answer: An air conditioner that runs continuously without lowering the temperature has lost the ability to move heat out of the house fast enough, and the cause is almost always one of nine things: a clogged filter, a condenser coil packed with pollen, a failed run capacitor, a frozen evaporator coil, low refrigerant, leaking ductwork, blocked registers or returns, a thermostat problem, or a system that is simply too old or too small for the house.

On this page:

• Do this right now

• Match the symptom to the cause

• The nine causes, in order

• What you can fix and what needs a technician

• Urgent tonight or fine until morning

• Why quotes differ and what a leak repair involves

• FAQs

It is ninety-four degrees in the greater Atlanta area, the thermostat says seventy-two, the system has not shut off since lunch, and the house is holding at eighty-one.

The good news is that the two most common causes on that list are things you can check in the next ten minutes without tools. The rest are not, and knowing which is which saves you a service call or saves you a compressor.

Do This Right Now

1. Set the thermostat to OFF and the fan to ON. If the indoor coil or the refrigerant line outside has iced over, this thaws it without running the compressor into damage.

2. Pull the filter and hold it up to a light. If light will not pass through it, replace it before you do anything else.

3. Walk out to the outdoor unit and clear leaves, pollen mats and grass clippings off the fins. Keep two feet of clear air on every side of it.

4. Check that every supply register and every return grille in the house is open and not blocked by furniture or a rug. 

Stop here and call if the outdoor unit is buzzing with a stationary fan blade, or if the breaker has tripped twice. Both conditions damage the compressor if you keep resetting them, and a compressor is the most expensive part of the system.

Match the Symptom to the Likely Cause

Find your situation in the left column to work out how urgent it is before you pick up the phone.

The Nine Causes, in the Order We Actually Find Them

These are the failures behind most no-cooling calls in the greater Atlanta area, ordered by how often they turn up on a truck rather than by how interesting they are. 

Clogged Air Filter

A filter dense with dust starves the evaporator coil of airflow, which drops coil temperature until the coil ices and airflow stops entirely. It is the single most common cause and the only one on this list you can fully resolve yourself. 

During pollen season here, a filter that normally lasts three months can load up in one. 

Pollen and Cottonwood on the Condenser Coil

Metro Atlanta's pollen season buries outdoor coils in a felt-like mat that blocks the unit from rejecting heat to the outside air. The system runs, the compressor works, and the heat has nowhere to go.

Kill power at the outdoor disconnect and rinse from the inside outward with a garden hose so debris is pushed out rather than driven deeper. A pressure washer bends the fins permanently and turns a cleaning into a coil replacement.

Failed Run Capacitor

The capacitor gives the compressor and fan motors the jolt they need to start. When it weakens, the motor tries and fails, which is the buzzing sound with a fan that will not turn.

This is a genuinely small, common part and an inexpensive repair when caught quickly. Left humming, it takes the compressor with it.

Frozen Evaporator Coil

Ice on the indoor coil or on the refrigerant line is always a symptom rather than a cause, and the cause is either restricted airflow or low refrigerant. Running the system while it is frozen can slug liquid refrigerant back to the compressor.

Thaw it first with the fan setting, then find out why. We covered the process in detail in what to do if your AC freezes up.

Low Refrigerant Charge

Refrigerant is not consumed. A system that is low has a leak, and a system that has been topped off twice has a leak somebody chose not to find.

Low charge shows up as long run times, weak cooling and often ice on the line set. It needs a licensed technician because refrigerant handling is federally regulated.

Leaking or Disconnected Ductwork

Attic ductwork that has come apart at a joint dumps conditioned air into a space that is already well over a hundred degrees in July. The system is cooling; it is just not cooling your living room.

This is also the usual answer when both the summer and the winter bills are high, because a duct leak costs you in both seasons. Related reading: why ductwork sweats.

Closed or Blocked Registers and Returns

Closing vents in unused rooms does not save energy on a central system. It raises static pressure, reduces total airflow and makes the rest of the house worse.

Blocked returns are the version of this people miss, because a sofa against a return grille is invisible until you look for it. If one room is the problem rather than the whole house, start with one room hotter than the rest of the house.

Thermostat Settings and Dead Batteries

Check that the mode is on COOL rather than on FAN, that the setpoint is actually below room temperature, and that the fan is set to AUTO rather than ON, because ON runs the blower continuously and makes the vents feel like they are blowing room-temperature air.

Replace the batteries if it is battery powered. It is a thirty-second check that occasionally ends the whole problem.

Undersized or Aging System

A system that never quite keeps up on the hottest days, has been repaired repeatedly, and is past fifteen years old is telling you something the next repair will not fix.

Undersizing is the other version: a system that was never matched to the house through a proper Manual J load calculation will run continuously in August no matter how well it is maintained.

What You Can Fix and What Needs a Licensed Technician

Three of the nine causes above are genuinely homeowner territory. The other six are not, and the reasons are specific rather than liability boilerplate.

You can do these today: change the filter, clear debris off the outdoor unit with the power off, open closed registers and returns, replace thermostat batteries and check the mode, thaw a frozen coil using the fan setting, and reset a tripped breaker once. 

Call a licensed technician for these: anything involving refrigerant, anything involving a capacitor or contactor, a motor that hums without turning, a breaker that trips a second time, water anywhere in an attic, and any burning smell. Refrigerant work is federally regulated, and a capacitor holds a dangerous charge after the power is off.

If you are in the second list, call 770-852-8504 and we will get a technician out. Casteel provides AC repair across the greater Atlanta area.

Urgent Tonight, or Fine Until Morning

Not every no-cooling call is an emergency, and knowing which one you have is worth something at nine in the evening.

Call now, do not wait:

• Water is running from an attic air handler onto a ceiling below.

• There is a burning or electrical smell from the air handler or the outdoor unit.

• The breaker has tripped more than once in a day.

• The outdoor unit is humming with a stationary fan blade.

• An infant, an older adult, someone with a medical condition, or a pet alone at home is in a house that is climbing past the mid-eighties indoors.

Safe to schedule for the next business day:

• The house cools, just more slowly than it used to.

• One room is warm while the rest of the house is fine.

• The system is noisy but still cooling.

• You found and replaced a badly clogged filter and things are improving.

For the first list, that is what emergency AC service exists for. While you wait, close blinds on the sun side, run ceiling fans in occupied rooms only, and move anyone vulnerable to the coolest part of the house.

Why Two AC Quotes Can Differ by Thousands

We cannot publish prices, and any page that gives you a number without seeing your house is guessing. What we can tell you is what drives the spread, because the equipment is rarely the reason.

A complete replacement quote should name the equipment and its efficiency tier, the Manual J load calculation it was sized from, the labor, the permit, removal and disposal of the old equipment, the thermostat, an evaluation of the refrigerant line set, any electrical work, duct modifications, and the written warranty terms.

Two quotes that differ sharply almost always differ on ductwork, line set replacement, permit handling and warranty rather than on the box in the yard. Ask each contractor to put those items side by side and the gap explains itself.

What a Refrigerant Leak Repair Actually Involves

A refrigerant leak is not a top-off. A proper repair means locating the leak, quoting the repair against the age and condition of the system, evacuating and recharging to the manufacturer's specified weight, and verifying superheat and subcooling afterward.

On an older system the real question is not what the leak costs but whether the system is worth repairing at all. Three answers decide it: where the leak is, how old the coil is, and whether your system uses a refrigerant that is still in production. 

Ask for all three in writing. If the answer points toward replacement, AC replacement is the conversation, and it should start with a load calculation rather than with a model number.

Why Choose Casteel for AC Repair in the Greater Atlanta Area

We diagnose before we quote, we show you what we found, and we tell you when the honest answer is that a repair is not worth making. That includes telling you when the fix is a filter.

Most of the nine causes above are preventable, and the two that cost the most rarely arrive without warning. Seasonal AC maintenance catches a weakening capacitor and a loading coil before August does, and the Comfort360 membership covers it on a schedule so you are not the one remembering.

If an attic air handler has already leaked once, ask about a drip pan sensor. It shuts the system down before the ceiling is the thing that tells you.

Call 770-852-8504 or book a free estimate.

AC Running but Not Cooling FAQs

Why is my AC running but not cooling the house?

An air conditioner that runs without cooling has usually lost the ability to move heat outside fast enough. The most common causes are a clogged filter, a condenser coil packed with pollen, duct leakage into the attic, low refrigerant, a failed capacitor or a frozen evaporator coil. Change the filter first.

My outdoor unit is buzzing but the fan won't spin, what is it?

A humming outdoor unit with a stationary fan blade is nearly always a failed run capacitor or a seized fan motor. Shut the system off at the thermostat and the outdoor disconnect right away, because letting it hum overheats the compressor and turns a small repair into a replacement.

My AC keeps tripping the breaker every afternoon, why?

Afternoon-only breaker trips point to a compressor drawing high current at peak heat, often behind a weakening capacitor, a dirty condenser coil or a failing motor. Reset the breaker once. If it trips again, leave it off and call, because repeated resets feed a fault that can destroy the compressor.

Water is leaking from my air handler in the attic, who do I call?

Call an HVAC technician the same day, not a plumber. Water at an attic air handler is almost always a clogged condensate drain line or a rusted-through secondary pan, and the ceiling below is what fails next. Shut the system off at the thermostat and catch the water while you wait.

My coils are covered in cottonwood fluff and pollen, how do I clean them?

Kill power at the outdoor disconnect, then rinse the condenser fins gently with a garden hose from the inside outward so debris is pushed out rather than deeper in. Never use a pressure washer, which bends the fins permanently. Heavy matting or indoor coil fouling needs a technician.

My AC died and quotes range widely, why are they so different?

The equipment is rarely the reason. Quotes diverge on duct modifications, refrigerant line set replacement, permit handling, electrical work, disposal and the written warranty term. Ask every contractor to list those items separately alongside the Manual J load calculation, and the difference usually explains itself.