An air conditioning contactor failing to pull in or engage when your indoor thermostat calls for cooling is typically caused by a lack of 24-volt low-voltage control power or a burnt electromagnetic coil. Common culprits include a tripped condensate float switch, a blown low-voltage fuse on your indoor furnace control board, or a severed control wire running from the thermostat to the outdoor condensing unit. Additionally, if the contactor's internal electromagnetic coil has suffered an open circuit or internal short due to a power surge, it will lack the magnetic force required to pull the plunger down. You can troubleshoot by testing for 24V across the small coil terminals with a digital multimeter while the system is calling for cooling. You can verify that the issue is resolved when proper voltage reaches the coil, causing it to click and close the contact points instantly.
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