[AT&T Samsung Galaxy S 5] lordcheeto03:
I do apologise if I give too much information; I try to be thorough in documenting my methods and include as much information as possible so we can maybe skip some of the "what does it do when you ..." and the "did you try this" questions.

So I have been reading through these forums for the past few hours... Earlier today a friend gave me her AT&T Galaxy S5 to try and fix. When I received it, it would boot straight into recovery. She was on version G900AUCS4DQD1. I can say for nearly 100% certain that this lady knows nothing about modding her devices, so I am absolutely confident that this problem doesn't stem from her attempting to root the 6.0.1 firmware she was on, install custom recovery, or anything anywhere close to that. She said she had taken it to an AT&T Store and they were unable to fix the device; although I'm sure all they did was do a factory reset through recovery and when that didn't work they just said "Well, it's broke, come buy a new device." Either way, the information in bootloader mode doesn't show anything outside of what it's supposed to ... Current Binary is Samsung Official, System Status is Official, Reactivation Lock is off, and most importantly the KNOX Warranty Void flag is showing 0x0.

As I said, when I got it, the phone would boot straight to recovery by only person the power button to turn it on; but download mode was accessible via the option in recovery and by holding Vol-Down + Home during boot. Attempting to boot into safe mode, holding Vol-Down during boot results in nothing different than if I don't hold Vol-Down... sometimes it boots all the way to system where it reboots on Optimizing Apps, and sometimes it doesn't even clear the colorful animated Samsung Boot logo. While in recovery, the device is not visible in Windows Explorer. If I open my command line and type "adb devices" the S5 doesn't how up. However, if I select "Apply update from ADB", THEN type adb devices, it shows up as <device serial #> sideload. Attempting to issue the command "adb shell" while visible to adb results in "error: closed" being returned. Now, because I've been using the OnePlus 3T I have become somewhat familiar with bootloader mode and the fastboot command, and I don't know if Samsung devices are SUPPOSED to respond to the command "fastboot devices" while in bootloader/download mode, but I can say for 100% certain this one doesn't.

Remembering everything I learned when I owned the S4 AND after doing research on what firmware versions I could safely flash if my device was on 6.0.1, the first thing I went searching for was Odin and the factory firmware. I used the thread "G900AUCS4DQD1 ** FULL ODIN UPDATE ** v6.0.1 by toutanje to find the firmwares to download. I have also used the method and files in the thread "[Unbrick] Fix All For Samsung S5 AT&T (SM-G900A) Lollipop 5.1.1 by Birkyjon. Whichever method I use, all the files load and flash without incident in Odin. Before I attempt to boot to system, I boot into recovery and wipe cache and do a factory reset. (I have attempted do boot both with and without a factory reset after flashing the firmware in Odin) After doing that, obviously I hit reboot now... the phone passes the initial Galaxy S5 Logo, it goes to the multicolored Samsung Logo, then the AT&T screen, and SOMETIMES it boots into system, where it sits at the "Optimizing Apps, x out of 210." It will count up to around 30-40 or so, then reboot. After that initial boot where it gets to Optimizing Apps, all boots after that are unpredictable... as in, maybe it will make it back to Optimizing Apps, maybe it won't even make it to the AT&T boot screen before it restarts itself. Once it does that, I can boot back to recovery, do another factory reset, and it will make it to Optimizing Apps again every time. About the only thing I haven't tried was to attempt to flash the firmware via the "Apply update from ADB" option, though I honest don't see the point after everything else which has failed.

One last thing I have seen in my research so far but am unable to test: could these issues be caused by a bad battery? Being that I have no idea how the battery life on this particular device has been, I have no reference as to whether it has been draining incredibly fast, acting normal, or even just flat out dropping massive amounts or so at once.. I seem to remember noticing when I placed it on charge this afternoon, the device only had around 6%, and while connected to only a 1A charging brick, the percentage had magically jumped from 6% to upwards of 35% in what seemed like 3-4 minutes. I can't confirm for certain how long it was charging as I was also wrangling my two toddlers at the time and could have possibly just lost track of how much time had actually gone by...

Any ideas as to the issue I am facing? Any help would be much appreciated! Thanks guys!