Quick answer: When the AC is running but not cooling, check the easy things first: make sure the thermostat is set to “cool” and “auto,” replace a dirty air filter, confirm both breakers (inside and outside) are on, and clear anything blocking the outdoor unit. If the outdoor fan isn’t spinning, the indoor coil is iced over, or the air from the vents is barely cool after those fixes, you likely have a failed part or a refrigerant leak that needs an HVAC technician.
It’s 95 degrees, the AC is humming, and the house keeps getting warmer. Before you call anyone, know this: a good share of “no cooling” calls turn out to be a thermostat setting, a clogged filter or a tripped breaker. Those take five minutes to fix.
This guide walks through the usual reasons for an AC running but not cooling, starting with the checks you can do yourself and ending with the problems that need a licensed technician. Work through them in order.
Start here: 5-minute checks
With an AC running but not cooling, these five checks solve a surprising number of cases.
1. Thermostat settings
It sounds too simple, but check it first.
- Make sure the system is set to Cool, not Heat or Fan.
- Set the fan to Auto, not On. When the fan is set to On, it keeps blowing even when the AC isn’t cooling, which can push warm air through the vents between cycles.
- Set the temperature at least 3 degrees below the current room temperature.
- If the display is blank or dim, replace the batteries.
2. A dirty air filter
A clogged filter chokes airflow. Your AC keeps running, but not enough air moves across the cold coil to cool the house. Over time, it can even cause the coil to freeze.
Pull the filter out and hold it up to a light. If you can’t see light through it, replace it. The U.S. Department of Energy recommends cleaning or replacing AC filters every month or two during cooling season and says a clean filter can lower an air conditioner’s energy use by 5% to 15%.
3. Tripped breakers or a switched-off disconnect
Central AC has two parts: the indoor unit (furnace or air handler) and the outdoor unit (condenser). If the indoor fan runs but the outdoor unit is silent, the outdoor unit may have lost power.
- Check your electrical panel for a tripped breaker labeled AC, condenser or similar. Reset it once.
- Check the disconnect box on the wall next to the outdoor unit and make sure it’s on.
If the breaker trips again, leave it off and call a technician. A breaker that keeps tripping points to an electrical or compressor problem.
4. A blocked or dirty outdoor unit
The outdoor unit gets rid of the heat pulled out of your home. If it can’t breathe, the system can’t cool.
- Clear away leaves, grass clippings, weeds and anything stored against the unit. Energy.gov recommends keeping plants trimmed at least 2 feet back.
- Look at the fins. If they’re matted with cottonwood fluff or dirt, turn off power at the disconnect and gently rinse from the inside out with a garden hose (no pressure washer).
5. Closed or blocked vents
Walk the house and make sure supply vents are open and not covered by rugs or furniture. Also check that return grilles aren’t blocked. Closing too many vents doesn’t save energy; it strains the system.
Problems to look for next
If the quick checks didn’t solve it, look for these signs. Some you can spot yourself; most need a pro to fix.
6. A frozen evaporator coil
Look at the refrigerant lines at the outdoor unit and around the indoor unit. Ice or frost on the copper line or coil means the evaporator coil is freezing. That happens when airflow is too low (dirty filter, blocked return, failing blower) or refrigerant is low.
What to do: Turn the AC off and set the fan to On to melt the ice, which can take a few hours. Replace the filter. If it freezes again after restarting, call a technician.
7. The outdoor fan isn’t spinning
If the outdoor unit hums but the fan on top doesn’t turn, the capacitor or fan motor has likely failed. Without that fan, the unit can’t release heat, and the compressor can overheat.
What to do: Turn the system off to protect the compressor and call for service. Capacitors store a dangerous electrical charge, so this isn’t a DIY repair.
8. Low refrigerant (a leak)
AC systems don’t use up refrigerant. If it’s low, there’s a leak. Signs include ice on the lines, a hissing or bubbling sound, air that’s only slightly cool, and higher electric bills.
What to do: Call a licensed HVAC technician. Only certified techs can handle refrigerant, and a good one will find and fix the leak rather than just recharging the system.
9. Leaky ducts or an undersized system
If the AC cools some rooms but not others, or has never kept up on the hottest days, the problem may be leaky ductwork in the attic or crawlspace, or a system that’s too small for the house. Attic ducts that leak dump cooled air where it doesn’t help you.
What to do: Ask an HVAC contractor about a duct inspection and a load calculation (often called a Manual J) to see whether the system is sized right.
How hot is too hot? What your AC can realistically do
Most central air systems are designed to cool the house about 15 to 20 degrees below the outdoor temperature. On a 105-degree afternoon, a house at 80 to 85 degrees may be the system doing its best, not a breakdown.
A quick test: set your thermostat, wait 15 minutes, and use a kitchen thermometer to compare the air going into a return grille with the air coming out of a nearby supply vent. A healthy system usually shows a difference of roughly 15 to 20 degrees. Much less than that points to a real problem.
Common AC repair costs
Every home and system is different, but these ballpark ranges help you know what to expect. National price guides put most AC repairs between about $450 and $2,000, with a new capacitor or thermostat at the low end and compressor or coil work at the high end. If your system is more than 10 to 15 years old and needs a major repair, get a replacement quote too.
When to call an HVAC technician
Still have an AC running but not cooling after the checks above? Call a pro, especially if:
- The outdoor fan or compressor won’t run, or the breaker keeps tripping.
- You see ice on the coil or lines that returns after thawing.
- You hear hissing, grinding, squealing or loud clicking.
- The air from the vents is warm or barely cool after you’ve replaced the filter.
- Water is pooling around the indoor unit.
- The system is short cycling, turning on and off every few minutes.
Tell the technician what you already checked. It saves diagnostic time and can lower your bill.
How to keep it from happening again
- Replace the filter every one to three months, and monthly during heavy use or if you have pets.
- Book a tune-up each spring, before the first heat wave.
- Keep the outdoor unit clear and rinse the fins once a year.
- Pour a cup of white vinegar down the condensate drain line in spring to prevent clogs (check your manufacturer’s guidance first).
- Replace an old manual thermostat with a programmable one so the system isn’t fighting huge swings.
Frequently asked questions
Why is my AC running but not cooling the house?
The most common causes are a thermostat set wrong, a dirty air filter, a tripped breaker for the outdoor unit, a blocked or dirty condenser, a frozen coil or low refrigerant from a leak. Start with the filter and thermostat, then check the outdoor unit.
Why is my AC blowing warm air?
Warm air usually means the outdoor unit isn’t running. Check the breaker and the disconnect box. If the fan outside isn’t spinning, a failed capacitor or fan motor is likely, and you should turn the system off and call a technician.
Can a dirty air filter stop my AC from cooling?
Yes. A clogged filter restricts airflow across the cold coil, so the house cools slowly and the coil can freeze. Replacing a dirty filter is the single most common fix for weak cooling.
How do I know if my AC is low on refrigerant?
Warning signs include ice on the refrigerant lines, a hissing sound, air that’s only slightly cool and rising energy bills. Low refrigerant always means a leak, so have a licensed technician find and repair it.
Should I turn off my AC if it’s not cooling?
If the coil is frozen, the outdoor fan isn’t spinning or the breaker keeps tripping, yes. Running it can damage the compressor, the most expensive part of the system. Turn it off and call for service.
How long should it take my AC to cool the house?
A healthy system usually lowers the temperature about 1 degree every 15 to 20 minutes in normal conditions. On very hot days it takes longer, and most systems can only hold the house 15 to 20 degrees below the outdoor temperature.
Why does my AC cool some rooms but not others?
Uneven cooling usually comes from closed or blocked vents, leaky or poorly designed ductwork, or rooms that get heavy afternoon sun. An HVAC contractor can test airflow and inspect the ducts.
Can I recharge my AC refrigerant myself?
No. Handling refrigerant requires EPA certification, and adding refrigerant without fixing the leak only delays the problem. A licensed technician should find the leak, repair it and charge the system correctly.
How often should an AC be serviced?
Once a year, ideally in spring before cooling season. A tune-up includes cleaning coils, checking refrigerant levels and electrical parts, and clearing the drain line, which helps prevent mid-summer breakdowns.
When is it time to replace my AC instead of repairing it?
Consider replacement when the system is more than 10 to 15 years old, needs a major repair such as a compressor, uses outdated refrigerant, or has needed several repairs in a few years. Get quotes for both repair and replacement to compare.
Still not cooling? Get an HVAC pro out today
If you’ve checked the thermostat, filter, breakers and outdoor unit and your AC is still running but not cooling, it’s time for a professional diagnosis. Find HVAC contractors near you on We Got Your Guys, compare reviews and licenses, and call them direct.