AC running but not cooling: 6 checks before you call

What to do, in order

1. Check the filter — a clogged filter starves the system. Replace if you can't see light through it. This fixes a surprising share of 'not cooling' calls.
2. Confirm the outdoor unit is running. If the fan's dead: check the breaker, then the disconnect box by the unit.
3. Feel the big copper line at the outdoor unit: cold and sweaty is good; warm means refrigerant issues or a compressor problem.
4. Ice on the indoor coil or copper lines: turn the system to FAN-ONLY and let it thaw completely (hours), with a fresh filter in. Restart in cooling — if it ices again, low charge or airflow, tech time.
5. Check thermostat settings: COOL mode (not FAN), set point well below current temp. A dying thermostat lies.
6. Close enough supply registers? Blocked returns choke the loop — keep returns unobstructed.

Red flags — stop and call a pro

• Burning smell or buzzing from the outdoor unit — capacitor/compressor failing; running it makes damage worse, shut it off
• Ice + warm house after one thaw cycle — refrigerant leak; continued running can slug the compressor ($1,500+ mistake)
• No air at all from vents with the blower running — duct collapse or blower failure

Prevention

Filters every 60–90 days (30 with pets), annual spring tune-up, and keep the outdoor unit's fins clear of lint and shrubbery by 2 feet.

This guide is maintained by the ee.ltd editorial team — reviewed for 2026. Browse all home repair guides or compare hvac costs.

Related guides

• Furnace short cycling (starts and stops every few minutes)
• AC evaporator coil frozen: why it happens and the thaw protocol
• Thermostat not working: dead screen, wrong temps, no response
• AC won't turn on: what to check before you call

What this repair typically costs

• AC repair cost guide
• AC replacement cost guide
• Furnace replacement cost guide
• Heat pump installation cost guide