Chosen Solution
After a cracked screen and water damage, I replaced screen and battery of this iPhone 6S. Got through ifixit’s water damage guide. Now there is no issues as long as phone screen stays on(auto lock never). But if I lock the phone or autolock does, after 3 or 5 minutes phone not wakes up and not working, just black screen, hold power on button does nothing. Only solution is to hard reset phone with home+lock button. I’m suspicious about screen so ordered a new screen and try with that but any thoughts are welcome. Update (06/27/2019) New screen has this issue as well, so problem is within the logic board or connectors due to water damage. Are there any background app that will constantly keeps CPU awake at minimum state? That way this phone will work.
After a cracked screen and water damage…That unfortunately, is the problem. Water damaged phones are always a crap shoot. Even the tiniest amount of water can find it’s way into the smallest and hardest to reach areas. If you didn’t remove the shields on the logic board, then you are not seeing all the potential damage. Even without visible damage, shorts caused by the water can cause upstream and downstream damage that can be hard to identify and repair. At this point, you would have to troubleshoot at the component level to figure out, if possible, what the issue is.
