AC Not Turning On

Most common cause. The breaker tripped from a power surge, overload, or short. Some outdoor units also have inline fuses in the disconnect box.

Go to your electrical panel. Look for AC breaker — it may not look fully OFF, just slightly out of position. Flip it fully off then back on. Outside, open the disconnect box — small cartridge fuses may be visible. Test with a multimeter (or have a pro do this safely).

If the capacitor failed completely, you'll hear a hum from the outdoor unit but no fan or compressor will start. Sometimes the fan tries to start and stops.

Stand by the outdoor unit while someone calls for cooling at the thermostat. Hum but no spin = capacitor. Manually pushing the fan blade with a long stick (carefully, power off!) sometimes starts it briefly — confirms capacitor.

The contactor is the relay that supplies high-voltage power to the compressor when the thermostat calls for cooling. Years of use cause the contacts to pit and weld together, or fail open completely.

Visual inspection (power OFF!): contactor is a 2-3 inch block with copper connections. Look for burned/pitted contacts, signs of arcing, or melted plastic. With a multimeter you can verify 24V across the coil and 240V across the load terminals when the thermostat calls.

Thermostat is dead, batteries are flat, or the wiring is loose. Without a call for cooling, the AC never gets the signal to start.

Is the display lit? Replace batteries if dim/blank. If display works but no AC response, jumper R to Y at the thermostat wires (safely — short to call for cooling). If AC starts, thermostat is bad. If still no response, problem is downstream.

A safety float switch in the condensate drain pan has detected water and shut the system down to prevent water damage. Common cause: clogged condensate drain line.

Find your indoor air handler. Look for the condensate drain pan and the float switch (small plastic device with wires). If water is in the pan or the switch is in the UP position, it's tripped.

Someone (lawn care, painters, electrician) turned off the outdoor disconnect and forgot to turn it back on. Sometimes pulled-out fuse-style disconnects are reinserted upside down (non-functional).

Open the gray box on the wall next to the outdoor unit. Verify the switch is in the ON position, or the fuse pull-out is fully inserted in the correct orientation.

Failed compressor, broken low-voltage wiring, or burnt transformer. The system has no path to start.

Beyond DIY diagnosis. Symptoms include: breaker trips immediately, smell of burning electronics, no 24V at the contactor when thermostat calls. Requires a multimeter and HVAC knowledge.