Confirm the call for cooling
Set the thermostat to cool and lower the setting a few degrees below the room temperature. Wait several minutes. Some systems use a built-in delay after a power interruption, battery change, or previous short cycle.
Homeowner Guide
Thermostat says cool but the AC is not running? Start with what is actually responding.
When the thermostat says cool, the problem is not always the thermostat. The AC may be waiting on a delay, missing a control signal, shut down by a safety switch, or failing to start outside. A few simple observations help Scott separate a thermostat problem from an AC diagnostic.
Notice which part of the system responds
If the indoor blower runs but the outdoor unit stays quiet, the issue may be outside, at the disconnect, in a control circuit, or with an outdoor component. If neither the indoor nor outdoor unit responds, the thermostat, furnace or air handler power, drain safety, fuse, board, or wiring may need to be checked.
Read the outdoor unit startup guideCheck safe basics without forcing the system
Make sure the display is on, the thermostat mode is cooling, the filter is not badly restricted, and the breaker has not tripped. If a breaker trips again, the outdoor unit hums without starting, water is around the indoor unit, or anything smells hot, stop testing and request a diagnostic.
Read the breaker-trip safety guideWhen this should be a diagnostic
A thermostat that says cool but does not start the AC can involve controls, wiring, drain safety, airflow, outdoor electrical parts, or equipment protection logic. The $95 Springfield Diagnostic is the right first step when the system will not respond normally.
Read the diagnostic page