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The iphone screen is very dark. Can hardly see the icons. Increasing brightness doesn't work. Everything else is working fine. Any help with a diagnosis? thx.

You have a broken backlight, meaning the LED lamp that lights your screen from the back is broken, or it is not powered by the logic board.
Try a screen replacement first, if that doesn't fix it, it's a logic board. If it got wet, cleaning out the waterdamage may get it to work again.