Why does my Xbox One S turn off immediately?

@xdstarr definitely some water damage. First thing you need to do is to clean that board. Remove the heatsink etc from it and get some high grade (+90%) isopropyl alcohol. Poor it in containing large enough for your motherboard and then soak the board. Use a couple of soft brushes and "gently" clean the board. You do not want to scrub it since that potentially could knock components of the board. You can use something like this guide Even so it was written for an iPhone logic board, all the points are still pertinent to your board as well. While you are cleaning it, you need to look for any burned, charred or missing components. Those can be replaced and traces can be fixed if needed. Post some good, clear, large pictures with your QUESTION after it's cleaned so that we can see what you see. Pay particular attention around area D9 on your board since that is where the power IC's etc. are located. Adding images to an existing question