Downgrading from Android 5.1.1 to Android 4.1

[marcsmv3]

I actually didn't have an external SD card. I wound up taking it to the ATT store that had an ATT repair shop inside it and they reset it back to stock 4.4 for me and now everything works great! After unlocking the carrier on the phone I did have to manually enter the t-mobile APN to get the data connection working but that was simple (the info is listed on t-mobile's site)

Sorry to bump an old post but I am in the exact same boat as you were with an i747U AT&T GS3 and I have a custom ROM called LiquidSmooth (LS-LP-v4.1-2015-07-13) which based on 5.1.1.... now I am trying to perform a network unlock on my phone so I can use a different SIM card which I need because I am about to do some overseas traveling... It asks for 'SIM network unlock PIN' when booting with a new SIM card (MaxRoam) but no code I have works: AT&T provided unlock code (8 digits) doesn't work and the Maxroam provided unlock code doesn't work (4 digits)... it says unsuccessful attempt and there is no attempt counter that forces a PUK request after 3 attempts (I do I have the Maxroam provided PUK as well), it just lets me make infinite attempts.

My bootloader is I747UCDLK3
My baseband is I747UCUEMJB (I remember flashing this in order to get the antenna to work properly)
My kernel is 3.4.108-Deathly-Rebirth Deadman@Deadman #1 Mon July 13 01:00:58 EDT 2015

How can I downgrade to a stock Android ROM such that I can properly unlock the phone to use other SIM cards? Stock means 4.4.2, doesn't it? But apparently I need to get all the way back to 4.1.1 in order to use the AT&T provided unlock code. How can I do that without bricking the phone? Is this ROM compatible with my baseband/bootloader: http://xdaforums.com/showthread.php?t=2161667 ?? Or if I take the phone as-is to an independent phone shop will they be able to perform a network unlock some other way or find a proper 'SIM network unlock PIN' that will work when I put in another SIM card? Please help this amatuer, I'm not a programmer I'm just relatively computer literate!

 

[audit13]

First, I would use twrp to back up everything, including the efs folder.

Since the Bootloader is older than 4.3, you can use Odin to flash back to a stock 4.1.1 rom, use the free method to sim unlock your phone or try the codes, and then flash twrp and restore your twrp backup.

Stock 4.1.1 is needed to use the free method to sim unlock the phone. A stock Samsung dialler is required to enter a network unlock code. Any stock rom can be used to enter a network unlock code, not just 4.1.1.

 

[marcsmv3]

I cannot get Odin to connect with my phone. What if I flashed the ROM I linked (rooted stock 4.1.1) using TWRP? I'd clear data and dalvik and cache first of course. Then after I did the network unlock I should be able to get to TWRP via recovery mode (right? Since i used a rooted stock ROM?) and load a backup or flash a new ROM like maybe the 6.0.1 based one I see out there (Oct-M or something?).... Right?

---------- Post added at 02:38 PM ---------- Previous post was at 02:35 PM ----------

How would you suggest doing it without Odin?? And if 4.1.1 is not necessary for network unlock what is the version I should be downgrading to??? Please point me to the proper ROM I dont want to screw my only phone up...

 

[audit13]

If you want to flash the ROM you linked via TWRP, Odin is not required. All you have to do to flash the ROM you linked is either copy it to the phone's internal or external memory, wipe the system and cache, flash the ROM, wipe data, reboot, unlock, restore backup.

The only version you can flash via Odin is from sammobile.com and it happens to be 4.1.1: http://www.sammobile.com/firmwares/database/SGH-I747/

The only reason you can flash 4.1.1 is because you are running an lk3 bootloader; otherwise, you'd never be able to use Odin.