The system sounds alive, air moves, but the house keeps warming. Techs diagnose this in a specific order — cheapest and most common first — and you can walk the first two-thirds of it yourself.
1. Filter and airflow
A choked filter can cut airflow enough to mimic a refrigerant problem. If the filter looks like a lint blanket, replace it, and while you're there make sure most supply vents are open. This fixes more "not cooling" complaints than any part does.
2. Look for ice
Check the copper lines at the outdoor unit and the indoor coil if accessible. Ice anywhere means shut the cooling OFF and run the fan only for a few hours — a frozen coil can't transfer heat, and running it risks the compressor. Ice traces back to airflow problems or low refrigerant.
3. The outdoor unit check
Is the big fan on top spinning? Is the coil around it packed with cottonwood or grass clippings? A dirty condenser coil can't reject heat — hose it down gently (power off). If the fan isn't spinning while the unit hums, you're at step 4.
4. The capacitor
Humming, slow-starting, or fan-not-spinning symptoms point to the run capacitor — the most common failure in AC. The 60-second capacitor test settles it; the part is $15–$40.
5. Refrigerant — the one that needs a license
If airflow is good, the coil is clean, and everything spins but cooling is weak, low charge is likely — and that means a leak, because refrigerant doesn't get "used up." A tech will confirm with superheat and subcooling readings (the same math as our free charge calculator). On an older R-22 system, get the age off the nameplate before approving a recharge — our photo decoder reads it in seconds — because the repair-or-replace math often flips at that point.
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