1. Check the thermostat
Make sure it’s set to COOL and the temperature is set a few degrees below the room. If the screen is blank or dim, put in fresh batteries before anything else. After you change the setting, give it up to 5 minutes — most systems have a built-in delay before the compressor starts.
2. Check the power — both units
Air conditioning has two power points: the indoor blower (often powered through the furnace) and the outdoor condenser, which has its own breaker and its own disconnect box mounted on the wall next to it. Check both breakers, and make sure the disconnect is fully seated.
3. Check the filter
A badly clogged filter chokes airflow across the indoor coil until it gets so cold it freezes over — and once that happens, the system stops cooling even though it’s running. If yours is gray and caked, replace it and give the system time to thaw before running it again.
4. Look for ice
Check the indoor coil and the refrigerant lines running to the outdoor unit. Ice anywhere means stop — switch to Fan Only, not Off, so warm air can help it thaw. Running an iced-up system can damage the compressor, and it’s almost always a sign of low refrigerant or restricted airflow — a call-a-pro situation.
5. Check the outdoor unit
Is the outdoor fan spinning? If not, that points to a bad capacitor, contactor, or motor — don’t keep flipping the breaker to try again. Also clear away anything blocking airflow around it: grass clippings, leaves, or a fence line built too close can choke the unit's ability to reject heat.
6. Don’t keep resetting a tripping breaker
If the outdoor unit’s breaker trips again right after you reset it, that’s usually an electrical short, not a fluke. Repeatedly resetting it is a real fire risk — leave it off and call a licensed technician.
7. Listen before you keep trying
A hum or click with nothing starting is almost always electrical — a weak capacitor or a stuck contactor. It won’t fix itself by trying again, and repeated attempts can strain the motor further.
When to call a pro
If you’ve checked these and it’s still not cooling — or it’s icing up, tripping breakers, or blowing warm air with the fan running fine — that’s past a quick DIY fix. A lot of these no-cool calls also trace back to a worn part that a yearly AC tune-up would have caught early.
Prefer a printable version of the full checklist (furnace and AC)? Get the before you call checklist — save it to your phone’s Notes app for next time.
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