Code 51 is capacitor volt low on start up. Check for any load on the NO side of the line contactor. There should not be any accessory items connected. Also check that the PTC discs connected to the input output posts on the line contactor are connected. The PTCs should be warm to the touch with the battery plugged in. The PTCs are used to keep the capacitor in the master controller charged when the key is off. The capacitor supplies voltage to the controller during POST before the line contactor closes.