As Jon pointed out the boards can be easily replaced,but also the contacts that allow the inverter to be bypassed sometimes need to be cleaned.They are in the front of the inverter,I used 320,400,600 sand paper on mine,I cut the sandpaper into 3/8 inch strips in order to get around wires etc.Before taking the cover off be sure to disconnect shore and or generator and turn off the incoming 24 volt disconnect to the inverter.Mine would not invert but would bypass on shore or generator.