Air conditioner running but not cooling? Start here
The outside unit hums, the air is moving, and the house is still warm. Here is what that usually means, in the order it is usually true.
Ryan DeRue · September 14, 2026 · 1 min read
Running but not cooling is a different problem from not running at all, and it narrows things down more than people expect.
Check the outdoor unit fan
If the unit is humming but the fan on top is not spinning, switch the system off at the thermostat and stop there. That is usually a failed capacitor, and leaving it running while the compressor struggles against it is how a repair becomes a replacement.
Check for ice
Ice on the copper line or on the indoor coil means airflow or refrigerant, and it will not cool anything while it is frozen. Switch the cooling off, leave the fan on, and give it a few hours to thaw. Then change the filter, because restricted airflow is the most common cause of it.
Check the filter and the vents
A blocked filter starves the coil, drops the coil temperature, and freezes it. Closed or blocked supply registers do the same thing. Both are free to fix.
Check the condenser is not choked
Grass clippings, cottonwood and leaves pack into the outdoor coil and stop it rejecting heat. Switch the power off at the disconnect and rinse it gently from the inside out with a hose. Do not use a pressure washer, and do not bend the fins.
What is left
If all of that is clear, it is most likely low refrigerant, which means a leak. Refrigerant is not consumed. A system low on charge has lost it somewhere, and topping it up without finding the leak is paying twice for the same gas.
What we do on that call
Pressures and superheat measured against spec, capacitor tested under load, delta T measured across the coil, and a written price before anything is replaced.
Read it and still not sure?
Describe what the system is doing and we will tell you whether it is a five minute fix, a real repair, or a replacement conversation. That costs nothing.
Quick answers
How cold should the air from the vents be?
Around 16 to 22 degrees cooler than the air going into the return. That gap is called delta T, and it is the first thing a technician measures. Much less usually means low charge or an airflow problem.
Can I add refrigerant myself?
No. It needs gauges and a set of numbers to compare against, overcharging damages the compressor, and venting refrigerant is illegal. If it is low, it has leaked, and the leak is the actual problem.