Forcing an air conditioning unit to turn on when your thermostat fails or during diagnostic testing requires manually bypassing the thermostat control signals at the indoor furnace or air handler control board. First, turn off the main electrical power to your HVAC system at the circuit breaker panel for safety. Second, remove the indoor furnace access panel to locate the low-voltage terminal strip (labeled R, C, Y, G, W). Third, use an insulated jumper wire to manually bridge or connect the power terminal (R) directly to the cooling terminal (Y) and the fan terminal (G). Fourth, restore power at the circuit breaker. This manual jumper action bypasses the wall thermostat entirely and sends 24V power directly to the AC contactor. You can verify that this procedure is successful when the outdoor compressor and indoor fan immediately engage.
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