Quick answer
An air conditioner blows warm air when it can't move heat out of the house. The usual causes are a wrong thermostat setting, a dirty filter, a tripped breaker for the outdoor unit, a dirty outdoor coil, a failed capacitor or contactor, a refrigerant leak, or a frozen indoor coil. The first three are safe for you to check. The rest need a licensed technician.
Checks you can safely do yourself
Thermostat: set it to Cool and a few degrees below room temperature. If the display is blank, replace the batteries. A fan set to On instead of Auto will keep blowing air even when the compressor is resting, which feels warm.
Air filter: pull the filter and hold it up to a light. If you can't see light through it, replace it. Restricted airflow can make the indoor coil ice over, and then it stops cooling completely.
Breakers and disconnect: the outdoor unit usually has its own double breaker in the main panel and a disconnect box on the wall near the condenser. If a breaker tripped, reset it once. If it trips again, leave it off and call for service.
Outdoor unit: clear away leaves, plants, and anything stacked against it. The condenser needs open space to release heat.
Problems that need a technician
Capacitor or contactor failure: if you hear a hum from the outdoor unit but the fan doesn't spin, or nothing runs at all, a start capacitor or contactor has often failed. These parts store high voltage and should only be handled by a professional.
Refrigerant leak: signs include ice on the copper line, a hissing sound, and cooling that fades over weeks. A technician should find and repair the leak before recharging the system.
Frozen evaporator coil: turn the system to Off with the fan On to let the ice melt, then have the cause diagnosed. Ice usually points to low airflow or low refrigerant.
Dirty condenser coil: in Los Angeles dust and pollen, the outdoor coil can clog enough to raise pressures and cut cooling. Cleaning should be done carefully so the fins aren't bent.
When warm air is an emergency
During a heat wave, loss of cooling can be a health risk for infants, older adults, and anyone with a medical condition. If you smell burning, see smoke, or the breaker keeps tripping, turn the system off at the thermostat and the breaker and call right away. HVAC Nation keeps a 24/7 emergency line at (310) 906-8164.
