1. Thermostat first — the embarrassing fix is the best fix
Confirm the mode is COOL (not FAN or HEAT), the set point is at least 3°F below room temperature, and the display is alive. Dead batteries silently disable cooling on many models. If your thermostat is smart, check that a schedule or eco mode hasn't overridden you.
2. The filter — the #1 silent killer of airflow
A clogged filter chokes airflow, which can make the coil freeze and the system blow warm. Pull the filter and hold it up to a light: if you can't see light through it, replace it. Run the fan with the filter out for a few minutes only as a test — never long-term.
3. Look for ice — and thaw it the right way
Frost or ice on the indoor coil or the larger copper line means stop: cooling can't recover until it thaws. Set the system to FAN ON with COOL off for 2–4 hours. Running it iced can flood the unit and stress the compressor — an expensive part to sacrifice to impatience. See the full frozen coil guide.
4. Breakers and switches
Check the breaker panel for a tripped AC or condenser breaker (flip fully OFF, then ON). Outdoor units also have a service disconnect box on the wall nearby — make sure it's seated. Indoor air handlers often have a light-switch-style shutoff that gets flipped by accident.
5. The outdoor unit
Is the big fan on top spinning? Is the unit humming but the fan still? A humming-but-still fan often points to a failed capacitor — one of the most common and cheapest pro repairs. Clear leaves, cottonwood fluff, and debris at least 2 feet around the unit, and gently rinse the fins with a hose (power off first).
Sources
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