I had this strange problem. I would insert battery into Samsung Galaxy S4 and it would turn on and then shuts off after couple seconds. After some research on Google I found the power button is stuck and pressed all the time – ON.

On a brand new phone you insert battery and then you have to press the power button to turn it on.

On a broken phone the phone will turn on after you insert the battery and because the button is pressed on all the time it will turn itself off again.

SOLUTION: take the phone and put it sideways where the power button is facing down on a table or some flat surface and don’t slam it! but lightly hit it maybe a couple of times and the button will come unstuck. The phone should turn on by itself but the button isn’t stuck so it will turn on perfectly fine.