Safety first. If you smell burning or the breaker trips again right after a reset, turn the system off at the thermostat and the breaker and leave it off. That's an electrical fault.
Checks, in order
- Thermostat screen. Blank? Replace the batteries. Set it to Cool, lower the set point a few degrees and wait five minutes (many systems have a restart delay).
- Breakers. Find the AC and air-handler breakers. A tripped breaker can sit in the middle position: push it fully off, then on. Once.
- Outdoor disconnect. The box next to the outdoor unit. Some have a pull-out block that must be fully seated. Don't open it further if you're not comfortable with it: it's 240 volts.
- Float switch. Look at the pan under the air handler. If it's full of water, the float switch has shut everything down to prevent a flood. See AC leaking water.
- Air-handler door. Many units have a safety switch that cuts power if the panel isn't closed properly after a filter change.
What it probably is, if those are fine
- Failed thermostat or wiring, often after a surge or a DIY thermostat swap.
- Blown low-voltage fuse on the control board. Usually caused by a short somewhere, which has to be found too.
- Failed transformer or control board, common after lightning.
- Failed capacitor or contactor at the outdoor unit, if the indoor fan works but the outdoor unit doesn't.
When to call
If a breaker trips again after one reset, stop and call: that's an electrical fault. Otherwise, call if the checks above don't bring it back. Tell us what happens at the thermostat and whether anything runs at all.
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