AC Won't Cool? What to Check First

Before booking a repair, a few checks can rule out simple causes. Here's where to start.

Thermostat

Set to Cool, below room temperature, with good batteries. Check the schedule hasn't changed.

Filter

A clogged filter can stop cooling almost completely. Replace it if dirty.

Diagram: the filter size is printed on its edge. Fit it with the airflow arrow pointing toward the furnace or air handler.

Breakers

Check the breakers for the indoor and outdoor units. Reset once; if one trips again, get help.

Disconnect

Make sure the disconnect beside the outdoor unit is on.

Outdoor fan

With the system on, the outdoor fan should spin. A hum with no spin often means a failed capacitor.

Ice

Ice on the lines or coil means turn the system off and let it thaw, then check airflow.

Water

If the system stopped and there's water around the indoor unit, a clogged drain may have tripped a safety switch.

Diagram: condensation from the indoor coil collects in a drain pan and leaves through the condensate line. A clog makes the pan overflow.

Still stuck?

See AC repair.

Write it down

Note what the system was doing when it stopped, any error codes on the thermostat, and what you checked. A technician can often narrow down the fault from that information before they arrive.

When to stop and get help

If a breaker trips twice, you smell burning, or you hear loud grinding from the outdoor unit, switch the system off and leave it off until a technician has looked at it.

More AC guides

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