Why is my AC blowing warm air?

Air still coming out of the vents, but it is room temperature or worse. That symptom narrows things down more than you would think, because it means the blower is working — the problem is on the cooling side, not the air-moving side. Work through these in order.

1. Check the thermostat fan setting

Start here, because it costs nothing and it is genuinely common. If the fan is set to ON rather than AUTO, the blower runs continuously — including between cooling cycles, when it is just pushing unconditioned air around. Set it to AUTO and see whether the air turns cold when the system actually calls for cooling.

2. Look at the filter, then look for ice

A badly clogged filter starves the evaporator coil of airflow, and a starved coil freezes into a block of ice. Once that happens, air passes over ice instead of through it, and what reaches the vents is weak and not cold. If the filter is filthy or you can see frost on the refrigerant lines, turn the system off — fan only — and let it thaw for a few hours with a fresh filter in. If it cools normally afterward, the filter was the whole story.

3. Check the outdoor unit

• Is the outdoor fan spinning? If the indoor blower runs but the outdoor unit is dead silent, suspect a tripped breaker — outdoor units are often on their own.
• Reset a tripped breaker once. If it trips again immediately, stop and call a tech; something is drawing fault current.
• Clear leaves, grass clippings, and shrubs back about two feet — a smothered condenser cannot dump heat.

4. If all that checks out, it is likely refrigerant

Low refrigerant means a leak, because refrigerant is not consumed — a sealed system that is low has a hole somewhere. “Topping it off” without finding the leak just pays to refill the same hole next season. This is licensed-tech territory: handling refrigerant is regulated, and the diagnosis needs gauges.

One thing worth knowing: if the AC has been running hard and cooling poorly for weeks, and then quit entirely, the filter-and-ice story is the most likely explanation by a wide margin. It is worth ruling out before you book anything.