Short answer
When the AC won't start at all, check the thermostat display, the breaker, the outdoor disconnect, the furnace or air handler switch, and the condensate float switch. A blank thermostat often means a tripped float switch or a blown 3-amp fuse on the control board.
Thermostat
If the display is blank, replace batteries. If it's hardwired and still blank, the system's low-voltage power is off, often because of a float switch or blown fuse.
Power
Check both breakers (air handler and condenser) and the outdoor disconnect. Reset a tripped breaker once. If it trips again, stop.
Water safety switches
Check the drain pan and secondary pan under the air handler. Standing water means a clogged drain has triggered the float switch.
Call a tech for
Repeated breaker trips, a buzzing contactor, a burnt smell, or a blown fuse that blows again.
- Blown fuses point to a shorted wire or part
- Burnt smells mean shut the power off
- Hum with no start points to a capacitor
Questions
Where is the 3-amp fuse?+
On the control board inside the furnace or air handler. Turn off power before opening the panel, or have a tech check it.
Editorial note: this guide is general information for homeowners, not a diagnosis. Desertline Heating & Air is a referral service; a licensed contractor should inspect your equipment before repairs.