DIAGNOSIS The top cover interlock switch is faulty or the actuator that presses it is broken. This is a known weak point. The switch tells the logic board the cover is secure.
FIX The interlock switch (Brother part # SW4109001) needs replacement. It's located on the right side frame, activated by a plastic tab on the top cover. You must remove the right side cover and top cover to access it.
IF THAT DIDN'T WORK Check the plastic actuator tab on the top cover itself. It can snap off. If broken, you need a new top cover assembly, or you can sometimes carefully epoxy the tab back in place.
HEADS UP Do not bypass this switch by taping it down. The interlock prevents exposure to the laser and high-voltage areas during operation.