That's great.
Now for the sake of the masses, explain exactly what you did.
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I bought a phone from a friend and called Sprint Customer Care to make sure it wasn't lost/stolen. It wasn't but they still weren't able to activate it because they said my device wasn't supported by Sprint and was not a Sprint device. (It definitely was. It had a Sprint model #, Said Sprint when I booted up, and said Sprint on the back of my phone)
I tinkered with my phone and realized it was Un-rooted by being able to go into Odin download mode. You can go into download mode by simultaneously pressing the power button, home button and volume down key. If it works you are rooted. There may be other ways to check, but that's how I did it.
I was having trouble un-rooting my phone because many threads I came across said I couldn't Un-root my device with their method if I had Knox Warranty on my phone. Their methods were for older android versions. (They said it will brick my device if I did it that way.)
You can figure this out by going into download mode by simultaneously pressing the power button, home button and volume down key. If your device says Knox Warranty or Warranty Bit on the screen then you have the update. SO USE THIS METHOD TO UNROOT / RETURN TO STOCK.
I asked XDA and they gave me this handy little website that allows you to un-root and return to stock with instructions.
http://xdaforums.com/showthread.php?t=2738533
***BEFORE YOU DO THIS BACKUP ALL YOUR DATA WITH HOLO BACKUP SO YOU DONT LOSE ANYTHING***
First go download Holo Backup. Extract the folder in the zip file to your desktop:
Get it Here:
http://omegavesko.github.io/SimpleADBBackup/
It's a self contained ADB solution for backing up your apps etc. Instructions are on the web page. Pretty simple really. Make a backup(s) and store it in the Holo folder on your desktop.
The only other problem I ran into while using this method is downloading the .tar.md5 file. The website wouldn't load the Captcha for me to enter to begin the download. So I looked around and the other one I found was downloading WAY too slow.
SO I found and used this one:
http://www.androidfilehost.com/?fid=23487008491966461
It worked perfect.
Instructions:
1. Extract Download file
2. Put phone into download mode (power off, then power button + Volume Down + Home Button (website didn't mention holding the home button but you have to with the S3), then volume up when prompted)
3. Open Odin (right click > open as administrator)
4. Load File L710VPUCND8_L710SPRCND8_L710VPUCND8_HOME.tar.md5 into the PDA slot
5. Start flashing
6. Grab some popcorn because it will take a while
The only thing they forgot to mention and it left my phone stuck on the Samsung boot screen is you must go into Recovery Mode (power off, then Power button + Home + Volume up) and CLEAR THE CACHE.
Reboot your device
Once it boots up call Sprint or talk to a representative online to activate your device and they will walk you through it.
VOILA, YOU ARE GOOD TO GO.