[T-Mobile, Samsung Galaxy SIII] TronMinecraft:
Hello, I know there are many threads on restoring a bricked S3, some of them for hard bricks, and some for soft bricks. None of the guides I found by scouring these forums provided all of the answers I needed.

INFO PLEASE READ ALL

Here I hope to outline how I restored my T999 Tmobile S3 to stock firmware after the 4.3 MJC update, and after bootloader bricking the phone. I do assume you are capable of using ADB, ODIN and TWRP. I believe this method will work for other S3 models, but I have no ability to test it. If it will work, firmware and debrick IMGs will have to be found for the specific device. The T999 firmware and IMGs will NOT work with other devices. This will restore to stock UVUEMJC 4.3 firmware. Will not be rooted, but existing 4.3 roots for the device should work.

I AM NOT RESPONSIBLE FOR ANY DAMAGE TO YOUR DEVICE. THE TOOLS USED IN THIS GUIDE HAVE THE POTENTIAL TO BRICK, OR OTHERWISE DAMAGE YOUR DEVICE. USE AT YOUR OWN RISK

So as context, I had been flashing different ROMS; then I updated to stock 4.3 UVUEMJC Firmware. This updated the bootloader, and installed the Samsung KNOX security upgrade. Both the bootloader and the KNOX are problems regarding rooting, rom installation, and downgrading (which I still have not achieved)

After upgrading, I installed a vanilla rootbox. I then attempted to flash the UVDMD5 bootloader at mnasledov's thread here: http://xdaforums.com/showthread.php?t=2282603

Lacking my present understanding, of the 4.3/KNOX problems, I managed to brick my phone by effectively attempting to downgrade the bootloader. Phone appeared to be hard bricked.

I tried many things to fix the phone, these are what worked:

GUIDE:

1. Get a Class 10 MicroSD card and follow KAsp3rd's guide to boot from the SD card: http://xdaforums.com/showthread.php?t=2439367

You should now be able to boot to download mode, recover, and system if it was undamaged when you bricked your phone.

2. Boot into download mode and install TWRP via ODIN

3. Boot into your new TWRP recovery. Do an ADB sideload of the KNOX removal script found in Kaito95's thread here: http://xdaforums.com/showthread.php?t=2507931 Check the clear cache and clear dalvik recovery boxes. You have to do an ADB sideload, because your SD card is in use as a bootloader.

4. Use ODIN with Download Mode to Re-Partion by flashing the PIT file found in invisiblek's downloads here: http://xdaforums.com/showthread.php?t=1848267

The Re-Partition box will have to be checked, and the PIT file loaded under Re-Partition. This is what I believe corrected the download mod fails I had been getting.

5. Use ODIN and Download mode to flash the original UVUEMJC firmware found in DocHoliday77's thread here: http://xdaforums.com/showthread.php?t=1949687

6. In TWRP do a factory reset, and a fix permissions. Reboot into system, (without the SD card)

If all went well you should be in business!

If you see any error in my guide, or have another correction, please let me know. If I have posted this in the wrong section I'll be happy to move it.

I hope this helps you.

ROOTING:

So after fixing my phone via the steps above, I still wanted to root it. To root it I simply ODIN flashed mrRobinson's rooted/deKNOXED 4.3 stock rom from here: http://xdaforums.com/showthread.php?t=1771687

It seemed very stable to me.

I then decided to use TWRP to install the CyanogenMod 10.2. If you choose to install CM, I believe you must use the 10.2 branch, because it is 4.3 based. Prior versions are not 4.3, and won't work with the 4.3 bootloaders. You can find the appropriate CM download by selecting your device here: http://download.cyanogenmod.org/ The T999 is d2tmo.

EDIT: YOU CAN RUN OLDER VERSIONS AFTER UPDATING TO 4.3.

This was pointed out to me by Adreaver. I personally am using rootbox 4.2.2 after having done the 4.3 official update. Any rom for the device should work, but certain lines may need be removed form the updater script. I am still looking into this.

NOTES

This has been confirmed to work on the T999L, but you need a different PIT file, and different firmware for the model. See [MENTION=4189586]mrspock08[/MENTION] 's post on page 2 of this thread for more info on the T999L

THANKS

I could not have debricked my phone or created this guide without referencing and using the work of several fine members of this forum. Particular thanks to:

[MENTION=3090489]mnasledov[/MENTION] for providing a thread with information about bootloaders.

[MENTION=3505796]KAsp3rd[/MENTION] for providing a way to boot from the SD card, enabling the entire rest of the debrick.

[MENTION=4840888]Kaito95[/MENTION] for the deKNOX script.

[MENTION=2385005]invisiblek[/MENTION] for the PIT file for correctly repartitioning the phone.

[MENTION=4081205]DocHoliday77[/MENTION] for the MJC 4.3 Stock Firmware mirror.

[MENTION=3355398]mrRobinson[/MENTION] for the rooted MJC Stock firmware

[MENTION=4686120]Adreaver[/MENTION] for correcting me that you can run an older version of android

And the threads of many many others who's work I read through while learning about how all of this works.