I recently purchased a broken S4 with a 'flickering screen' issue. Normally, people are told to return the phone for a replacement, but I can't do that.
In more detail, the problem begins after using the phone continuously for several minutes. The phone works flawlessly regardless of the broken glass screen, but after a while, begins to flicker. Only slightly at first, but then more and more severely until the screen shuts off. Turn the screen on and off again, and it's back to normal. (you normally need to leave the phone for a while before turning it back on, or it'll quickly just flicker itself off again). This to me suggests an over-heating problem, but I can't be sure.
I've wired the phone up the the TV using cheap cable, and the flicker doesn't show on the TV. That is, the phone starts going mental, doing its flicker thing, but the image on the screen remains constantly smooth when swiping - no flicker.
So... is this an image with the digitizer or the cable attaching the digitizer to the motherboard? If the screen doesn't flicker on the TV, that means we can rule out a motherboard issue, correct?
I just want to know what part of the phone to replace really