AC running but not cooling: a troubleshooting guide
When the AC runs but does not cool, check in this order: thermostat settings, the air filter, ice on the coil or lines, whether the outdoor unit is actually running, and the condenser coil. Low refrigerant, a failed capacitor or compressor, and leaky ducts need a technician. Call 833-877-1717 to request an independent AC repair provider.
RepairNowUSA editorial review · Last substantively reviewed October 13, 2026 · Review standards
Step 1: thermostat
- Mode set to "Cool", not "Heat", "Auto" with odd settings, or "Fan".
- Fan set to "Auto", not "On". With "On", the fan keeps blowing between cooling cycles, so the air can feel warm.
- Set temperature a few degrees below room temperature.
- Replace batteries, and check that a schedule or hold is not overriding you.
Step 2: air filter and vents
A clogged filter is one of the most common causes of poor cooling, and it can make the coil freeze. Replace it if it is gray or dusty. Make sure supply vents are open and nothing blocks the return grilles.
Step 3: check for ice
Look at the copper lines at the outdoor unit and, if you can, the indoor coil. Frost or ice means the coil is too cold, usually because of poor airflow or low refrigerant.
- Set the thermostat to "Off" and the fan to "On" to thaw the coil. This can take several hours.
- Replace the filter.
- Watch for water as the ice melts; the drain pan can overflow.
- Restart cooling. If it ices again, call for service.
Step 4: is the outdoor unit running?
With cooling on, go outside. The large fan on top should spin, and you should hear the compressor running.
- Nothing running outside: check the outdoor breaker and the disconnect box next to the unit.
- Humming but the fan does not turn: often a failed capacitor or fan motor. Turn the system off at the thermostat so the compressor does not overheat, and call for service. Do not poke the fan or open the panel; capacitors hold a dangerous charge.
- Fan runs but no compressor sound and the copper lines are not cool: a capacitor, contactor or compressor problem.
Step 5: outdoor coil
The outdoor coil gets rid of heat. If it is matted with dirt, grass clippings or cottonwood fluff, cooling suffers. With the power off at the disconnect, rinse the fins gently from the inside out with a garden hose. Keep two feet of clearance around the unit.
Step 6: measure the temperature drop
A rough check: after the system has run for 15 minutes, measure the air temperature at a return grille and at a nearby supply vent. A drop of roughly 15–20°F is typical for a working system. Much less than that points to a refrigerant, airflow or compressor problem. This is a rule of thumb, not a diagnosis.
Problems that need a technician
- Low refrigerant: refrigerant does not get used up, so low means a leak. A good technician finds and repairs the leak rather than just adding refrigerant.
- Capacitor, contactor or fan motor failure: common, often affordable repairs.
- Compressor failure: a major repair; ask about replacement cost versus the age of the system.
- Leaky or disconnected ducts: especially in attics. Rooms far from the unit stay warm.
- Undersized system: the AC runs constantly on the hottest days but cannot keep up.
When you call 833-877-1717, RepairNowUSA looks for a participating independent AC provider in your area. The provider confirms timing and price with you.
How RepairNowUSA can help
Call 833-877-1717 and describe what is happening and the property ZIP code. RepairNowUSA looks for a participating independent provider for that kind of work in your area and connects you if one is available. The provider confirms timing, scope and price with you and does the work. Availability and response times vary and are not guaranteed.
Call 833-877-1717Common questions
Why is my AC running but blowing warm air?
Common causes are the thermostat fan set to "On", a clogged filter, a frozen coil, an outdoor unit that is not running (often a capacitor or breaker), a dirty condenser coil, or low refrigerant from a leak.
Can I reset my AC?
You can reset a tripped breaker once. Some systems also have a reset on the thermostat or a float switch in the drain. If the breaker trips again or the system will not start, call for service.
How long should it take my AC to cool the house?
It varies with outdoor heat, house size and insulation. A working system usually lowers the temperature steadily; if it runs for hours with almost no change, something is wrong.
Is it safe to keep running an AC that is not cooling?
Running a system with a frozen coil or a humming outdoor unit that will not start can damage the compressor. Turn cooling off and call for service.
Can I call any time?
Yes. You can call 833-877-1717 at any time, including nights, weekends and holidays. Whether an independent provider can come out right away depends on your ZIP code, the time and provider capacity, so availability is not guaranteed.
Authoritative references
These public sources give general safety context. They do not replace an on-site evaluation, and RepairNowUSA does not perform the work.