AC Running But Not Cooling? 5 Checks Before You Call HVAC

Updated April 2026 · 6 min read

Your AC is humming along but the house is getting hotter. Before you call an HVAC technician — who charges $150–$250 just to show up plus parts — run through these five checks. They fix about 80% of "not cooling" problems and cost nothing or nearly nothing. Do them in order.

Check #1: The air filter (40% of cases)

A clogged filter restricts airflow across the evaporator coil, which makes the coil ice over, which stops all cooling. Filters should be changed every 1–3 months. If you can't remember the last time you changed yours, this is almost certainly your problem.

• Find the filter — usually in the return air grille or inside the air handler. Slide the old one out.
• Note the size printed on the frame (e.g., 16×25×1). Get a matching MERV 8–11 filter from Amazon or Home Depot — they're $5–$15 each.
• Slide new filter in with the arrow pointing toward the unit.
• If the old filter was gray or covered in dust, run the AC for 20 minutes and check if cooling is restored.

Check #2: Iced coils (20% of cases)

If you skipped filter changes for a while, or if airflow was otherwise restricted, the evaporator coil inside your air handler freezes into a block of ice. At that point, the AC will blow warm air even though the compressor is running.

1. Turn the AC off entirely at the thermostat. Don't just switch to fan.
2. Switch the fan to ON to circulate indoor air and help thaw.
3. Wait at least 4 hours (overnight is safer).
4. Change the filter while you're waiting.
5. Turn AC back on. If it blows cold, you're fixed. If it freezes again within a day, you have a refrigerant leak — pro job.

Check #3: Tripped breaker or outdoor disconnect (10% of cases)

Modern homes have two power sources for an AC:

• A double-pole breaker in the main electrical panel.
• A pull-out disconnect switch in a gray box on the outside wall near the condenser.

If the outdoor unit isn't running at all (fan not spinning, no hum), check both. Breakers can trip silently — flip fully off, then on. If it trips again immediately, stop and call HVAC — electrical issue.

Check #4: Dirty outdoor condenser (15% of cases)

The condenser is the big unit outside. Its fins collect grass clippings, leaves, cottonwood fluff, and dirt — which blocks the airflow that dumps heat outside. Without that airflow, the AC can't cool.

1. Shut off power at the outdoor disconnect.
2. Hose it down from inside out — spray water from inside the unit outward through the fins. A bottom-up angle helps. Don't use a pressure washer; you'll bend the fins.
3. Remove any plants or debris within 2 feet of the unit.
4. Straighten bent fins with a fin comb if you have one. Otherwise, leave slightly bent ones alone.
5. Turn power back on and run for an hour to test.

Check #5: Thermostat settings (5% of cases)

Sometimes the fix is embarrassingly simple:

• Thermostat set to HEAT instead of COOL (happens every spring).
• Fan set to ON instead of AUTO — the fan runs constantly but the AC only kicks in when needed, so warm blown air feels like the AC is broken.
• Dead thermostat batteries — check for a low battery indicator.
• Thermostat set temperature higher than current room temp.

When to call a professional

• Refrigerant issues — smells sweet, ice keeps forming, or you can hear hissing. Only licensed techs can legally handle refrigerant.
• Compressor not kicking on even though the fan runs.
• Electrical burn smell or breaker trips repeatedly.
• AC is over 12 years old and still not cooling after filter + coil check — may be time to replace.

Frequently asked questions