Chosen Solution
Hello. I have a personal Iphone 6s that I decided to buy a new housing which has back glass but unfortunately alot of things didnt sit flush (camera, aux connecter had to be cut etc) so I decided to return the product and move all the stuff back to its original housing. I moved all the stickers etc and the logic board doesn’t seem to have had any damage. But when I connected the display and turned the phone on (after connecting the battery last ofc, and disconnecting the battery first out of anything) the display has alot of lines or sometimes wont even turn on. But the black light still turns on. I played with it a little and when i have the screen in an odd position the display works perfectly fine. The connecters on the logic board seem fine, aswell as the LCD connecters seem fine. My guess is the FCP flex cable is broken due to too much bending. Any help would be appreciated.
It’s a possibility about the cable but another possibility is a bad connection. Unhook the screen and make sure the pins are in good condition and clean the connectors with a qtip and alcohol and a soft brush. Update (07/14/2020) It sounds like the screen was damaged. I would try another screen.