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The short answer: if the system runs but the house stays warm, measure the temperature split — the difference between air at the return grille and air at a supply register after 15 minutes of running. A healthy system reads 16 to 22°F. Under 14°F means restricted airflow or low refrigerant. Over 25°F means airflow is choked and the coil is icing. Check the filter and the outdoor coil first; those two account for most no-cooling calls, and both are free to fix.
We do not test or physically handle the equipment we write about — our product rankings come from aggregated owner reviews and expert consensus, explained in how we rank. The wider category sits at the heating and cooling hub.
How do I tell an airflow problem from a refrigerant problem?
One measurement separates them. Put a probe or a cheap kitchen thermometer in the return grille, put a second one in the nearest supply register, and let the system run 15 minutes. That difference is the temperature split, and it is the number an HVAC technician takes before anything else. Under 14°F with strong airflow at the registers points to refrigerant. Under 14°F with weak airflow points to a filter, a blocked return, a collapsed flex duct or a dirty coil. Over 25°F is a restriction that is starving the coil.
What are the most common causes, in order?
Four problems cover the large majority of no-cooling calls, and they fail in a predictable order of frequency. Two of them you can resolve yourself in ten minutes. The other two need a technician, and trying to shortcut them usually costs more than the repair.
| Symptom | Likely cause | What to do |
|---|---|---|
| Weak airflow, warm air, filter gray | Clogged filter starving the coil | Replace the filter, run 30 minutes, re-measure the split |
| Ice on the copper line or indoor coil | Frozen evaporator | Thermostat OFF, fan ON, thaw 3 to 8 hours |
| Outdoor unit running, fins packed with debris | Condenser cannot reject heat | Power off at the disconnect, rinse the fins from inside out |
| Outdoor unit hums, fan blade will not spin | Failed run capacitor or contactor | Technician — capacitors hold a charge after power is off |
| Strong airflow, split under 12°F, no ice | Low refrigerant from a leak | Technician; leak search before any charge is added |
| Cools some rooms, not others | Duct leakage or a closed damper | Check registers and accessible duct joints |
Can a dirty filter really stop an AC from cooling?
Yes, and it is the single most common cause. The evaporator coil needs a steady stream of warm room air to absorb heat into the refrigerant. Choke that airflow and the coil surface drops below 32°F, condensation on the fins turns to frost, the frost seals the remaining air gaps, and the coil becomes a block of ice that moves no heat at all. The Department of Energy notes that replacing a clogged filter can cut an air conditioner’s energy use by 5 to 15 percent — before you get to the cases where it stops the system entirely.
The interval depends on filter depth, not on the calendar you were handed with the furnace. We break the real numbers down in how often should you change an HVAC filter, and the MERV side of it — including why a too-restrictive filter can create exactly this problem — in how often should you change a furnace filter.
What should I check on the outdoor unit?
Walk outside while the system is calling for cooling. You are checking three things in about a minute: is the fan on top spinning, is the large insulated copper line cold and sweating, and can you see daylight through the coil fins. A condenser wrapped in cottonwood fluff, grass clippings or dryer lint cannot dump heat outdoors no matter how well the indoor half works. Shut power off at the outdoor disconnect, then rinse the fins from the inside out with a garden hose at normal pressure — a pressure washer folds the fins flat and permanently reduces capacity.
If the fan is not turning but the unit hums, stop there. That is usually a run capacitor, it is a common and inexpensive part, and capacitors store a dangerous charge for a long time after the power is off. This is not a homeowner repair, and nothing about it involves opening or altering wiring yourself.
Does an AC need refrigerant topped up?
Not in normal service. A residential split system is a sealed loop — it does not consume refrigerant the way a car consumes oil, so a low charge always means a leak. EPA Section 608 requires certification to buy or handle regulated refrigerants, which is why there is no legitimate DIY version of this repair. A technician should find and fix the leak, then weigh in a measured charge; a shop that adds refrigerant without a leak search is selling you the same repair again next summer.
If your system is limping along on an aging R-22 unit, get the repair-versus-replace math done before spending on it. A ductless replacement is often the sensible answer for a single problem room, and the current installed price ranges are in how much does it cost to install a mini split. In the meantime, a portable air conditioner or a well-placed ceiling fan buys you time, and a fan genuinely helps — see do ceiling fans actually cool a room for what it can and cannot do.