One of the Door Ajar Switches on the door latches themselves is either not working or is sticking. The easiest way to find out which one is at fault is by hooking up a diagnostic tool and monitoring the switches. You will have one (or more) doors show as "open" when they are all shut. This will direct you to the switch at fault. You could also spray some lubricant (WD-40, or equivalent) in the door latch area on each door and occasionally this will free up the switch to operate properly again.