AC blowing warm air: 6 causes ranked by cost

What to do, in order

1. Confirm the thermostat is on COOL (not FAN) and the setpoint is 3+ degrees below room temp.
2. Check the outdoor unit: fan spinning? Silence means power, capacitor, or contactor. Ice on the refrigerant lines means a freeze-up — turn the system to FAN-ONLY and let it thaw 2–3 hours before restarting.
3. Check the filter and replace if dirty — low airflow freezes coils and mimics low refrigerant.
4. If the outdoor fan runs but air stays warm for 30+ minutes, suspect refrigerant — look for oily residue on lines or the coil (leak signpost).
5. If the unit short-cycles (runs 5 min, stops, repeats), note the pattern — it narrows the diagnosis fast.
6. Get quotes with the diagnosis in writing: repair vs refill-and-find-leak vs replace.

Red flags — stop and call a pro

• Hissing or bubbling sound at the indoor coil — active refrigerant leak, call for same-day service
• Ice covering the outdoor unit in summer — shut it off to protect the compressor
• Burning smell from any vent — shut down at the breaker
• Warm air plus tripping breakers — compressor failing

Prevention

Annual tune-up with refrigerant charge check, keep filters clean, and keep the outdoor unit clear of lint and leaves — dirty coils raise head pressure and cause leak-inducing stress.

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

Related guides

• AC running but not cooling: 6 checks before you call
• 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

What this repair typically costs

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