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In each of the doors there is a unit that contains two motors, one of the motors locks the door and the other unlocks the door.
These motors can and do fail, if you have the patience of a saint, a photographic memory and the fingers of an origami expert you can open the unit and change the internal motor that has failed.
By far the easiest way is to purchase a known good or new unit and replace it. It involves removing the door card and is easier if you also remove the door glass.