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First of is there already a wire in Y slot? Y controls compressor in outdoor unit, so most likely nothing to do with your issue. Disconnect all of the thermostat wires at the furnace control board and see if fan still runs and let me know if we have a flashing light on control board in furnace.We will go from here.
OK if fan still runs with thermostat wires disconnected at furnace board and no error code, most likely a bad relay on board.We will go from here.