An AC that runs but doesn't cool is one of the most misdiagnosed HVAC problems. The compressor and fan can be humming along perfectly while the actual cooling process is completely broken — and it's rarely one thing you can eyeball. Here's the diagnostic order every good tech follows, so you can knock out the easy causes yourself before paying for a service call, and know when to stop DIYing before you make things worse.
Before anything else, confirm these three. About a third of no-cool calls end here without any parts changing hands:
A filter caked with dust starves the system of airflow. The evaporator coil freezes over, the compressor has nothing to cool, and warm air blows from the vents. Pull the filter — if you can't see light through it, replace it ($15 to $30). Then shut the system off for 4 hours to let the coil thaw before restarting. Filters should be changed every 60 to 90 days in most homes, and monthly if you have pets.
If the copper line coming out of the indoor unit is coated in ice, or you see frost on the coil itself, the system is frozen. Turn the thermostat OFF (not COOL), set the fan to ON to help thaw, and wait 3 to 6 hours. Then replace the filter and restart. If it freezes again within 24 hours, you likely have low refrigerant — that's a pro visit, no DIY workaround.
Refrigerant doesn't get 'used up' — a low charge always means there's a leak somewhere. Symptoms: cool air is barely cool, coil freezes repeatedly, hissing near the outdoor unit. This is not a DIY fix. In 2026 the EPA still restricts refrigerant handling to certified techs, and R-410A costs $80 to $200 per pound wholesale. A leak search plus recharge runs $350 to $900.
The outdoor unit dumps heat by blowing air across an aluminum coil. Wrap it in cottonwood fluff, grass clippings, or dryer lint and it can't shed heat — so the system runs constantly and cools poorly. Shut power off at the disconnect, spray the coil gently from inside out with a garden hose (never a pressure washer), and clear a 24-inch perimeter around the unit. Do this once a season.
Capacitors are the number-one electrical failure in central AC. Symptom: outdoor fan doesn't spin (or hums and stops), and you hear a clicking every few minutes. A tech will swap the capacitor for $150 to $350. DIY replacement is possible but the capacitor stores a lethal charge — if you haven't done it before, hire it out.
The contactor is the electromagnetic switch that turns the compressor on. Pitted contacts prevent the compressor from starting. Symptom: outdoor fan spins but you never hear the compressor kick on. Repair is $150 to $300 and takes about 30 minutes for a tech.
The compressor is the heart of the system and the most expensive part. On a unit under 10 years old with a valid warranty, replacement is $1,200 to $2,800 (mostly labor and refrigerant recovery). On a unit older than 10 years, or one out of warranty, replacing the whole outdoor unit — or the whole system — is almost always the smarter move.
If your AC has always struggled on 95°F+ days, it may be undersized for the square footage or leaking through undersized/uninsulated ductwork. This isn't a repair — it's a design problem. Have a tech do a Manual J load calculation before you replace anything, because just swapping in a bigger unit without fixing the ducts will underperform in the exact same way.
If the compressor tripped or you shut the system off, wait at least 5 minutes before restarting. Short-cycling the compressor — turning it on and off rapidly — is one of the fastest ways to kill it. If you thawed a frozen coil, wait 3 to 6 hours after all visible ice is gone before restarting so you don't refreeze it immediately.
Usually one of three things: airflow is restricted (dirty filter, blocked return, frozen coil), refrigerant is low from a leak, or the outdoor condenser coil is dirty and can't shed heat. Work through the list in that order — the first two account for probably 70% of 'not cold enough' calls in the summer.
Turn it up 5 to 8°F when you're gone, not off entirely. A completely off house heats up fast (especially the walls and furniture), and the AC works much harder to cool everything back down than to just maintain a slightly warmer temp. A smart thermostat that raises the temp during work hours saves 8 to 15% on cooling bills.
Once a year in spring, before you actually need it. A tune-up runs $80 to $175 and includes cleaning the outdoor coil, checking refrigerant charge, testing capacitors, and clearing the condensate drain. Skipping annual service is the biggest reason systems fail at age 8 to 10 instead of lasting 15 to 20 years.
No — or at most, cover only the top with a plywood square to keep debris out. A full-wrap cover traps moisture and invites rodents to nest inside where they chew wiring. The unit is designed to sit outdoors year-round; snow and ice do not harm it, but rodent nesting absolutely does.