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Nexus 4 flashing red light follows motherboard
Bottom line (details omitted): I have a Nexus 4 motherboard that gives the flashing red light from either of two Nexus 4 bodies. Remove SIMs to avoid confusion. Nexus A works fine (aside from broken glass). Nexus B gives flashing red light. Swap motherboards. A-body B-motherboard gives flashing red light. B-body A-motherboard works fine, and has Nexus A's password, apps, data, etc. Data from B-motherboard seems irretrievable.
By red flashing light, I mean: Hold power for 12 secs with good charger connected gives flashing red light: one short, pause circa 2 sec, 11 flashes at one per sec, pause under 1/4 sec, one short dim flash, pause c. 2 sec, repeat. This is the notification LED directly above the USB socket.
Motherboard is a single part with all connectors disconnected. There is nothing left to take apart. (Well, actually, I did transplant the rear facing camera as well). The body contains the battery, gorilla glass and LCD display and front facing camera assembly, USB connector assembly, speaker and WiFi antenna assembly, and the back. Maybe I forgot to mention something.
Additional probably irrelevant details: Nexus A has cracked gorilla glass, so you can't actually use it except for doing things which need only the power and volume control buttons.
Problem originally arose because of a flaky charger / usb plug that would only stay charging if you held the connector in a certain way. (Probably physical damage to plug). Also, I had the developer option "never sleep while charging", which probably defeated a protection against total discharge. It was working, but battery was getting low. I left it plugged in, and next morning it was dead. Gives red flashing as described above.
I was reading using Kindle Android app, so I switched to my Kindle Fire to finish the book, same charger, and the Kindle Fire also ended up discharged flat. Took wire cutters to that charger. Restrained myself from using sledge hammer. Discarded the pieces. I suspect that was the original Kindle Fire charger. Why don't they label them? Good charger recharged Kindle Fire easily.
Swapped batteries between broken glass Nexus A and red flashing Nexus B. The flat battery-B started charging in Nexus A. Nexus B continued to flash red even though battery-A is known to be (partly) charged, since Nexus A would boot to lock screen and wait for unlock gesture.
Swapped USB connector assemblies. USB-B works in Nexus A giving charge indicator. USB-A does not improve Nexus B.
Finally swapped the motherboards: put battery-A, USB-A, motherboard-B, and other parts-A (I think) in body-A, and vice versa.
Ended with body-B containing motherboard-A and memories of Nexus-A working fine, although I haven't tested things like camera, place phone call, but WiFi works (lots of apps got updated and web browsing works).
Body-A with broken glass and motherboard-B gives flashing red light.
Any ideas what this means? Maybe the number of flashes is a diagnostic code? Any way to get Nexus-B data?