So, recently, I've got myself a few broken Nexus 5 phones to try to repair them. One of these, however, eludes my attempts to diagnose/fix the problem.
A few minutes after i press the off button to turn the screen off, the phone ceases to react. Experimentally, I found out it was about two minutes. Before that limit, pressing the button again wakes the phone ok, everything works. After that time, the phone doesn't work anymore, and requires long button press to force restart.
It doesn't happen if the phone is connected via usb to charger/pc, no freeze whatsoever, only hangs on battery.
I've limited the scope of the problem to the mainboard - after transplanting it to different phone, same thing happens.
OS version doesn't matter, tested on KK, Lollipop, and Marshmallow, original ROM's - all the same.
Once when I connected frozen phone to PC, I entered a "DBI Apps Watchdog Reset! DemiGod Crash Handler" screen. (Not reliably reproducible behavior) Anybody has any idea what's going on and how to fix it? I have some experience with SMD soldering, so if it's something purely hardware related, I can attempt a fix myself. I also have a donor mainboard.