An air conditioner that won’t turn on with the thermostat set to Cool usually indicates a power, control, or safety lockout problem. The most common culprits are incorrect thermostat settings or dead thermostat batteries, followed by a tripped breaker or a condensate safety switch that shut the system down to prevent water damage. If the unit was just powered off, many systems impose a 3–5 minute restart delay to protect the compressor. Around our Trafalgar service calls, we often find a small algae plug in the drain pan float switch that silently stops the system.
Try these quick checks before calling:
- Set thermostat to Cool, Fan Auto, and drop the setpoint at least 5°F below room temp.
- Replace thermostat batteries and confirm the date/time isn’t in a schedule Hold.
- Check the furnace/air handler switch, outdoor disconnect, and the AC breaker; reset once only.
- Inspect the air filter; a severely clogged filter can trigger safety limits.
- Look for water in the indoor unit pan; a tripped float switch means the drain is clogged.
If a breaker trips again after one reset, leave it off and book service to avoid equipment or wiring damage.
Updated: 2026-06-05
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