Troubleshooting guide
AC running but not cooling? Start here
By the HeatAndAirCost editors · Last updated September 1, 2026 · How we research prices
Short answer
An AC that runs but won't cool is usually a dirty air filter blocking airflow, a tripped breaker on the outdoor unit, a frozen evaporator coil, dirty condenser coils outside, or a low refrigerant charge from a slow leak. The first four are free to check yourself. Refrigerant and electrical repairs need a licensed technician. Start with the free checks[1].
Try these first (cheapest fixes)
Work through this list before calling a technician. None of these steps require tools or a license.
- Confirm thermostat settings. Set it to COOL, fan to AUTO, and lower the target temperature at least 5 degrees below the current room temperature. A thermostat left in FAN or HEAT mode runs the blower without cooling.
- Replace a dirty air filter. A clogged filter is the single most common cause of poor cooling. It starves the evaporator coil of airflow, which causes the coil to freeze and prevents heat transfer. Check the filter first. If it is gray and packed, replace it.
- Check both breakers. Central AC systems have two: one for the air handler or furnace fan inside, and one for the outdoor condenser unit. Either can trip independently. Go to your electrical panel and reset any tripped breaker by switching it fully to OFF then back to ON.
- Clear the area around the outdoor condenser. The outdoor unit needs at least two feet of clear space on all sides to exhaust heat properly. Remove leaves, grass clippings, or anything else packed against the coil fins. You can gently rinse the fins with a garden hose from the inside out.
- Thaw a frozen evaporator coil. If you see ice on the refrigerant lines or the indoor coil (accessible behind the air handler panel), the coil is frozen. Turn the AC off, set the fan to ON only, and let it run for one to two hours to thaw. Replace the filter before switching back to COOL.
- Make sure vents and returns are open and unblocked. Closed supply registers or furniture pushed against return vents reduce airflow across the coil. Open all registers in the home and pull furniture away from any large return grilles.
What it usually means
If the free checks above didn't restore cooling, the system has a component-level problem that needs a technician:
- Dirty filter or restricted airflow (most common, free fix). Covered above. A frozen coil and a dirty filter are often the same problem.
- Frozen evaporator coil. Beyond a dirty filter, a coil can freeze because refrigerant is low (a leak) or the blower motor is weak. After thawing, if the coil refreezes within a few hours, refrigerant level is the likely cause and a technician needs to find and fix the leak.
- Dirty condenser coils. The outdoor unit rejects the heat pulled from your home. Coils packed with dirt, cottonwood, or debris cannot transfer heat efficiently. A technician can clean the coils with commercial coil cleaner.
- Low refrigerant (needs a pro). Refrigerant does not deplete normally. Low charge means a leak. Adding refrigerant without finding and repairing the leak is a temporary fix at best. Recharging a typical residential system costs $100 to $320[2], not counting leak detection and repair.
- Failed capacitor. The capacitor starts the compressor and outdoor fan motors. A weak or failed capacitor is one of the most common AC repairs, particularly on older units in hot climates. Replacement typically costs $150 to $400[1].
- Bad compressor. The compressor is the heart of the refrigeration cycle. A failed compressor means no cooling regardless of refrigerant charge. Compressor replacement is expensive and often tips the repair-vs-replace calculation toward a new system on units older than 10 years.
When it's urgent
- Burning smell from the unit. Shut the AC off at the thermostat and the breaker. A burning smell from an AC can indicate an overheating motor or an electrical fault. Do not restart it until a technician has inspected it.
- Breaker trips again immediately after resetting. A breaker that trips repeatedly is protecting you from a short circuit or ground fault. Do not keep resetting it. Call an HVAC technician or electrician. Refrigerant and electrical repairs inside the condenser are pro-only work.
- Water leaking from the air handler. A small amount of condensation is normal, but active dripping or pooling means the condensate drain is blocked or the coil is thawing from a freeze. Shut the system off and clear the drain line or let the coil thaw before calling for service.
- Outdoor unit making a grinding or clanking sound. Shut it off. A mechanical noise from the condenser usually means a fan blade or compressor part is failing. Running it in this state can cause further damage.
If you need a repair
See what a fair price looks like before you get a quote. Our AC repair cost guide covers capacitor replacement ($150 to $400), refrigerant recharge ($100 to $320[2]), and other common repairs. If the unit is older, our HVAC replacement cost page and the $5,000 rule guide can help you decide whether repair or replacement makes more sense.
People also ask
AC not cooling: FAQs
Why is my AC running but not cooling?
The most common causes are a dirty air filter restricting airflow, a frozen evaporator coil (also usually caused by a dirty filter or low refrigerant), dirty condenser coils on the outdoor unit, or a low refrigerant charge from a slow leak. Start with the filter and the condenser. If the indoor coil is iced over, turn the AC off and the fan to ON to thaw it, then replace the filter before restarting.
Can I fix my AC not cooling myself?
The free checks, yes. You can change the filter, clear debris from the outdoor unit, thaw a frozen coil, and confirm the thermostat settings and breaker without any tools or licenses. Refrigerant recharge and electrical repairs are a different story: refrigerant handling requires an EPA 608 certification and professional equipment, and electrical faults in the condenser or air handler carry real shock risk. Do the free checks first; call a pro for anything beyond that.
How much does it cost to fix an AC that's not cooling?
It depends on the cause. A capacitor replacement typically runs $150 to $400; a refrigerant recharge costs $100 to $320 for most systems. If the compressor has failed, repair costs rise significantly and replacement often makes more sense on an older unit. See our AC repair cost page for a full breakdown of common repairs.
Is it worth repairing an old AC unit?
Use the $5,000 rule: multiply the unit's age in years by the repair quote. If the result is over $5,000, replacement usually makes more sense than repair. A $300 capacitor repair on a 10-year-old unit scores 3,000 (repair); a $400 refrigerant repair on an 18-year-old unit scores 7,200 (lean toward replacement). See our full $5,000 rule guide for the complete framework.
Still not cooling?
Get a fair repair quote
If the free checks didn't fix it, a licensed technician can diagnose refrigerant, capacitor, and coil issues quickly. Know what a fair price looks like before you call.