[AT&T Samsung Galaxy S 5] shortydoggg:
UPDATE 03-06-2016: This is the latest and only FULL fix for the current Lollipop firmware: G900AATT4COI5 (Lollipop 5.1.1), which cannot be rooted at this point, and should fix all current versions of the At&T Galaxy S5, including Lollipop G900AUCU4BOF2 and G900AUCU4BOF3, but likely with the sacrifice of not being able to ever root again.

https://drive.google.com/file/d/0B25Y6zWgDo7yaF9BbnJfaDBmcm8/view?usp=sharing

You will need to unzip all of the files into a common folder. You need to boot into download mode. You have to flash the BL_G900A in BL, the AP_G900A file in AP, the CP_G900A file in CP, and the CSC_ATT_G900A in CSC. It's best to flash all four files at the same time. Make sure only Auto Reboot and F. Reset time are checked. Once the phone reboots after Odin, the operating system is updated to the latest firmware G900AATT4COI5. You will permanently loose root, however (if root is never found); and your phone will be factory reset because of the CSC_ATT_900A file.

To fix the stock partitions only, follow the thread by Birkyjon :

[Unbrick] Fix All For Samsung S5 AT&T (SM-G900A) Lollipop 5.1.1

http://xdaforums.com/att-galaxy-s5/general/how-to-update-to-g870aactive-modeloj3-5-t3247843

FOR FIRMWARE G900AUCU3BOC4:

Here is another link to the one-click file for the full firmware:

https://drive.google.com/file/d/0B25Y6zWgDo7yc0dOMWNfWS1sYXM/view?usp=sharing

Here also is the link to the fix the stock partitions only file for G900AUCU3BOC4 by muniz_ri:

https://www.androidfilehost.com/?fid=95916177934554805

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THIS AND EVERYTHING BELOW THE NEXT LINK IS OUTDATED AND ONLY WORKS FOR FIRMWARES G900AUCU1ANCE AND G900AUCU2AND3, AND MAYBE G900AUCU2ANG3.

For all firmwares 4.4.2 and 4.4.4, it's best to use the G900A_Downgrade_to_NCE.tar file by muniz_ri also found here:

https://www.androidfilehost.com/?fid=23622183712464416

These can be rooted again after downgrading.

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THE FORMER CORRECTED VERSION:

(Note this guide is mostly for Windows based computers and may not work on a Mac).

For those who've soft bricked their s5 after towel root, or just looking to downgrade (to enable the download booster fix for example, or just want to start over), and who are on firmware G900AUCU2AND3, this is a brief guide, as I don't have the time of day to make a full step by step guide.

1. You're going to need the firmware from this thread. Only use the first file download. The other, as well as the ones from Samsung-updates and Sammobile will not work.

http://xdaforums.com/showthread.php?t=2714455

or you can use the link from davew41 given here

http://www.mediafire.com/download/7np57282x5on1h1/SM-G900A+4.4.2+Firmware.rar

2. You'll need to extract these files. You'll likely need winrar to extract the files.

The .rar file contains 4 different .tar files The BL file is for the bootloader, the AP is for the boot image, recovery, and system, the CP is for the modem, and the CSC is for the cache.

3. You are going to need Odin 3.09.

http://www.mediafire.com/download/vwuqru9m1jp9wbw/Odin3_v3.09.zip

4. If you are doing a full downgrade, starting everything new with a factory reset, then you will need all of the files, except the BL file. BL for sure will not work, because you already have that bootloader.

You need to boot into download mode. You have to flash the AP_G900A file in AP, The CP_G900A file in CP, and the CSC_ATT_G900A in CSC. It's best to flash all three files at the same time. Make sure only Auto Reboot and F. Reset time are checked. Once the phone reboots after Odin, the operating system is downgraded back to the previous one. You will temporarily loose root (until you re-root), and your phone will be factory reset because of the CSC_ATT_900A file. The phone, minus the bootloader, will be in the exact state that it was in from the UCU1ANCE firmware, so that it is easily rooted again as well as upgraded back up again.

5. If you are looking to downgrade, but want to keep your userdata without a factory reset, then you will only need the AP_G900A file.

You only need to flash the AP_G900A file in AP. Make sure only Auto Reboot and F. Reset time are checked. Once the phone reboots after Odin, the operating system is downgraded back to the previous one. You will temporarily loose root (until you re-root), and your phone will not lose any of its userdata. The phone, minus the bootloader, will be in about the state that it was in from the UCU1ANCE firmware, so that it is easily rooted again. The disadvantage, if you want to call it that, is that the phone will not be upgradeable through OTA updates. That can be fixed at a later date by following step 4.

Note: If there was a factory reset before, or probably after this step, you might get some strange things in your operating system since you only flashed the AP file, like a different keyboard, and a mobile data toggle in the notification which I actually liked. Also, your APNs will probably get wiped and you will have to re-enter them manually:

Name: ATT Phone (Some might have PTA in place of Phone)

APN: phone

Proxy: Not set

Port : Not set

User name: Not set

Password: Not set

Server: Not set

MMSC: http://mmsc.mobile.att.net

MMS proxy: proxy.mobile.att.net

MMS port: 80

MCC: 310

MNC: 410

Authentication type: None

APN type: default,mms,supl,hipri,fota

APN protocol: IPv4

APN roaming protocol: IPv4

Enable/disable APN: APN enabled

Bearer: Unspecified

6. If you are looking to recover from a softbrick, but are not looking to loose your userdata, then your only need the AP_G900A file.

You only need to flash the AP_G900A file in AP. Make sure only Auto Reboot and F. Reset time are checked. Once the phone reboots after Odin, the operating system is downgraded back to the previous one. You will temporarily loose root (until you re-root), and your phone will not lose any of its userdata. The phone, minus the bootloader, will be in about the state that it was in from the UCU1ANCE firmware, so that it is easily rooted again. The disadvantage, if you want to call it that, is that the phone will not be upgradeable through OTA updates. That can be fixed at a later date by following step 4.

Note: If there was a factory reset before, or probably after this step, you might get some strange things in your operating system since you only flashed the AP file, like a different keyboard, and a mobile data toggle in the notification which I actually liked. Also, your APNs will probably get wiped and you will have to re-enter them manually:

Name: ATT Phone (Some might have PTA in place of Phone)

APN: phone

Proxy: Not set

Port : Not set

User name: Not set

Password: Not set

Server: Not set

MMSC: http://mmsc.mobile.att.net

MMS proxy: proxy.mobile.att.net

MMS port: 80

MCC: 310

MNC: 410

Authentication type: None

APN type: default,mms,supl,hipri,fota

APN protocol: IPv4

APN roaming protocol: IPv4

Enable/disable APN: APN enabled

Bearer: Unspecified

7. If you are are looking to recover from a softbrick, but want to start everything new with a factory reset, then you will need all of the files, except the BL file. BL for sure will not work, because you already have that bootloader.

You need to boot into download mode. You have to flash the AP_G900A file in AP, The CP_G900A file in CP, and the CSC_ATT_G900A in CSC. It's best to flash all three files at the same time. Make sure only Auto Reboot and F. Reset time are checked. Once the phone reboots after Odin, the operating system is downgraded back to the previous one. You will temporarily loose root (until you re-root), and your phone will be factory reset because of the CSC_ATT_900A file. The phone, minus the bootloader, will be in the exact state that it was in from the UCU1ANCE firmware, so that it is easily rooted again as well as upgraded back up again.

Thanks to davew41 and pcidiot for doing some of the things that I initially did not have time to do.

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THE ORIGINAL VERSION AND ORIGINAL EDIT: (so the previous comments won't become confusing)

For those who've soft bricked their s5 after towel root, and who are on firmware G900AUCU2AND3, including myself, we could all use the G900AUCU2AND3 firmware, which unfortunately is not available right now to the public.

I don't have the time of day to make a full step by step guide, but here are the steps that I did to get my phone back running. It's not perfect, and does not bring the phone back exactly to how it was, but it is an emergency fix for if you need the phone right away.

1. You're going to need the firmware from this thread. Only use the first file download. The other, as well as the one from Samsung-updates and Sammobile will not work.

http://xdaforums.com/showthread.php?t=2714455

2. You'll need to extract extract the files.*

3. You are going to need Odin 3.09. 3.07 may work as well. You will need the AP_G900A file only. BL for sure will not work, and I have not tried the other 2 files yet.

4. You need to boot into recovery and do a factory reset. Then you need to boot directly into download mode, and not into the operating system.

5. You have to flash the AP_G900A file in AP, or PDA with ODIN, depends on whether it's 3.07 or 3.09. Make sure only Auto Reboot and F. Reset time are checked.

Once my phone rebooted after Odin, the operating system was downgraded back to the previous one. I lost root access, as well as gaining some other things that I do not like, like the keyboard. I also gained a mobile data toggle for some reason (Maybe I had it previously, but don't remember it). I was able to easily re-root the phone, and I also had to re-enter my APN settings, as those were also wiped which killed my mobile data connection.

Like I said, it's an emergency fix. It's not perfect. If it does work for you, your phone for now will be stuck on the previous operating system until there is a ROM to flash or a new OTA, as there is no current firmware to flash, and the current OTA will also fail because you still have the same bootloader as the one you're trying to update to.

ORIGINAL EDIT: It has now been discovered that three of the four files, AP, CP, and CSC do indeed work. You have to flash them separately one at a time; flash one, reboot, then flash the next. You will temporarily loose root, and the last one, CSC, will factory reset your phone. Also the OTA now works after flashing all three files, so we are now all good.*

Thanks to davew41 and pcidiot for doing some of the things that I initially did not have time to do. I will re-edit this post later.