Haas Alarm: 649 (M) DC BUS UNDER VOLTAGE – DC Bus Undervoltage. The DC Bus voltage has dropped too low. This alarm occurs during charge up with low AC lines or after charge up when the bus falls below 100Vdc. Check the incoming AC lines to ensure nominal levels. If this fault occurs at power-up, it cannot be reset. If this fault occurs after power-up, it CAN be reset.