Symptoms
- No response from the system when set to Cool
- No sound from indoor or outdoor units
- Thermostat display may be blank or normal
Likely causes
- CommonThermostat set incorrectly or dead batteriesA thermostat that isn't calling for cooling won't start the system, even if everything else is fine.
- CommonTripped breaker for the indoor or outdoor unitAC systems typically have separate breakers for the air handler and the outdoor condenser.
- CommonOutdoor disconnect switch or fuse pulled or missingA safety disconnect near the outdoor unit can be off or have a missing fuse block.
- PossibleTripped condensate safety switchMany systems have a float switch that shuts the AC off if the drain pan gets too full, preventing damage.
- Less commonFailed control board or transformerInternal electrical failure can prevent the whole system from starting.
What to do
- Confirm the thermostat is set to Cool with a temperature below the current room reading, and replace batteries if needed.
- Check both the indoor and outdoor breakers at the panel, and reset any tripped ones once.
- Check the outdoor disconnect box for a properly seated fuse block or switch in the On position.
- Check the indoor unit's condensate drain pan for standing water, which can trip a safety float switch and shut the whole system down.
- Clear any drain clog as described for a leaking indoor unit if the float switch is the cause.
- If nothing responds after these checks, call an HVAC technician rather than continuing to reset breakers.
Tools and parts
- Flashlight
When to call a pro
Call an HVAC technician if the system still won't start after checking the thermostat, breakers, disconnect, and condensate switch.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.