Short answer
Start with the filter, thermostat and breaker. If the outdoor fan isn't spinning, the refrigerant line is iced, or the air from the vents is barely cooler than the room, turn the system off and call for service: those point to a failed capacitor, low refrigerant or an airflow problem.
Check these first (5 minutes)
Most no-cool calls that resolve themselves are found in this list.
- Thermostat set to Cool, fan on Auto, setpoint below room temperature
- Filter clean; a clogged filter can freeze the coil
- Breaker and outdoor disconnect switch on
- Outdoor unit clear of debris and running
- Condensate safety switch not tripped (water in the pan)
Common causes
If the basics are fine, these are the usual suspects, roughly in order of frequency.
- Failed run capacitor: outdoor fan won't spin, unit hums
- Dirty outdoor coil: system runs long, high head pressure
- Low refrigerant from a leak: ice on the suction line
- Frozen evaporator coil from poor airflow
- Clogged condensate drain shutting the system off
- Bad contactor: outdoor unit won't start
- Undersized or leaking ducts
When to turn it off
Running an AC with a stopped outdoor fan or an iced coil can destroy the compressor, the most expensive part. Switch the thermostat to Off (leave the fan on to thaw ice), and call.
What the technician does
Expect electrical tests on the capacitor and contactor, a refrigerant check by pressure and temperature, and an airflow measurement. A good tech finds a leak instead of just adding refrigerant, and tells you when the system's age makes repair a poor investment.
Common questions
How much cooler should air from vents be?
Typically about 15–20°F cooler than the air entering the return. Much less than that points to a refrigerant, airflow or coil problem.
Can low refrigerant fix itself?
No. Refrigerant isn't used up; if it's low, there's a leak. Topping off without a repair just delays the next failure.
This guide is general information, not a diagnosis. HVAC systems vary; a licensed technician should inspect yours before repairs.