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
Check the thermostat settings
A thermostat set to Heat, Off or a schedule hold will keep the AC off even when everything else works. Set it to Cool, lower the setpoint at least 3°F below room temperature, and wait five minutes: many systems have a built-in delay that protects the compressor after power is restored.
Outdoor unit runs but indoor fan doesn't (or the reverse)
If only one half of the system starts, the problem is narrower. An outdoor unit with a silent indoor blower points to a blower motor, capacitor or control board issue. An indoor fan with a silent outdoor unit points to the outdoor contactor, capacitor or a tripped outdoor breaker.
- Note which half runs before calling
- Listen for a click at the outdoor unit when cooling is called
- Don't open the outdoor panel; it carries line voltage
What to tell the technician
Giving a clear description saves diagnostic time: whether the thermostat display is on, which breakers you reset, whether there is water in the drain pan, and any blinking code on the indoor control board.
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.
Why does my AC take a few minutes to start after I change the thermostat?+
Many thermostats and control boards have a 3–5 minute anti-short-cycle delay that protects the compressor. Wait at least five minutes before assuming it has failed.
Can a power outage stop my AC from turning on?+
Yes. A surge can blow the control fuse or damage a capacitor, and some thermostats lose settings. Check the breaker and thermostat first, then call if it still won't start.
Is it safe to reset the AC breaker?+
Resetting a tripped breaker once is reasonable. If it trips again, leave it off: repeated trips point to a short, a failing compressor or a bad motor.
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.