You have to examine the logic board for damage/corrosion. Look for burned or swollen components. '''The only DIY test for logic boards is the Mark One Eyeball & a magnifying glass.'''
You might be able to pull the board and send it someplace to be examined with sophisitcated and very expensive machines… or just replace the logic board and screen. depends on how much you want to invest it repairing it..
Other option… part it out.
''If this answer is acceptable '''please remember to return and mark it'''.''