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Yes, to try if the screen is working alright you only have to connect the 2 flex cables from new screen to the logic board. No need to instal frontspeaker. (Face id is not attached to the screen, so to test new screen you don’t have to transfer frontspeaker to new screen)
I think you need to replace your screen with a new one, but before order a new screen you can open up the iphone and check if when you reseat the flex connectors the screen work properly.
I have given this answer already to someone, you can try this also and maybe this will help you to solve your problem. Because of it has been in contact with water you must clean the connectors with alcohol. Put a new good battery in it, that would be my first thing to do if it was mine. A few days ago I had a iPhone X what wasn’t recognise by iTunes and could not put it in DFU mode and the screen was black, no live at all. The iPhone was not recognise by iTunes because in settings the phone was disabled to use usb if it wasn’t used longer than 1 hour. Afther I put in a new battery the phone show a sign of live, then I put a new screen on the phone a now it works.
I think when you disconnect the battery connector you knock off a small component, take a magnifier and look around the battery connector on the logic board.
Put a new good battery in it, that would be my first thing to do if it was mine. A few days ago I had a iPhone X what wasn’t recognise by iTunes and could not put it in DFU mode and the screen was black, no live at all. The iPhone was not recognise by iTunes because in settings the phone was disabled to use usb if it wasn’t used longer than 1 hour. Afther I put in a new battery the phone show a sign of live, then I put a new screen on the phone a now it works.
It also is possible that face id just stop working even when the phone have never been dropped or came in contact with water or moister. Sometimes things just quit working because of poor quality, age or without a reason (like wear and tear). Just like a tv that stop working afther a few years. And if you want True Tone to work on afthermarket screen you have to program/copy serial no of the original screen to the afthermarket screen, it is not needed for a working Face ID.
If you connect the home button to your logicborad/phone does it respond to touch and go to home screen or when dubble touch it opens all the apps on background?
It can be a faulty battery or the battery connector on logicboard is comming off the logicboard. Check if the connector stay solid on logicboard and ther is no damage.
adhesive metal spacers