[Q] 4.3 back to 4.1.2 stock firmware - Galaxy S III Q&A, Help & Troubleshooting

Hey Guys,
First time poster.. so please don't flame me...eh!..LOL
I have searched and better searched without success to my question..
I would like to revert back to JB 4.1.2 on my S3 which is SIM locked to T-Mobile UK, would I need to ROOT the phone beforehand or can I just use Odin and the relevant stock firmware.
would this affect the IMEI and the EFS folder that would make the phone unusable?.
if the procedure is straightforward would I be able to revert back to perhaps 4.1.1 (which would allow me to unlock the SIM myself..).
Hope I have made some sense with my first post..
cheers for any help guys...

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4.3 changed your efs to v2. You cannot go back to 4.1.1 unless you have an efs backup from before you upgraded (an efs v1 backup).
Going back to 4.1.2 will work provided you use a firmware EMGx or later (efs v2).
Follow the guides in general stickies on how to flash stock using odin, root afterward with CF auto root.
There are many threads on efs changes, efs 'know this stuff' in particular.

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boomboomer said:
4.3 changed your efs to v2. You cannot go back to 4.1.1 unless you have an efs backup from before you upgraded (an efs v1 backup).
Going back to 4.1.2 will work provided you use a firmware EMGx or later (efs v2).
Click to expand...
Click to collapse
Do you mean this firmware I9300XXEMG4
cheers
cancel all that... I have managed to flash my s3 with the I9300XXEMG4 firmware..... and truth be told I was crapping myself when the phone kept looping on the logo screen.... I did a factory reset and cleared the cache.....all is working.....

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just a quick update...
My phone is working fine and dandy with the 4.1.2 stock firmware.... I have also discovered that the SIM is unlocked (or was not locked to begin with), tried a Tesco mobile SIM in it last night and it worked fine, was not asked to input unlocking code for it!
result all round..... :laugh:

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You help me too. Thx.

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So guys, please explain this to me.
After I did an update to 4.3 the efs was changed. I was ok with that, until my hard drive died and I had lost 4.1.2 backup and so, I can't restore my efs.
The problem is: Can i downgrade to I9300XXEMG4 without problems? I mean, I will not have IMEI and network signal problems? I did flashed some 4.3 roms, but I'm not satisfied with them.

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after effect in this downgrading process on my s3
I have upgraded from 4.1.2 XXEMH4 to 4.3 GMJ9 VDI first. liked that very much, display battery backup[battery doctor] was on my phone but drains quickly though in reality with lag issue. i found few things in 4.3:
1. time lag,
2. emulated drive
3. IMEI shows correctly but its SW ver shows 11 instead original 01
there were dozens of good features over above these aforementioned issues.
Sometimes wanted to use Samsung's Indian keyboard, downgraded therefore back to xxemh(4.1.2)..
Problems I found,
a. IMEI shows correctly with original SW version 01 but mobile shuts down suddenly quite frequently.
b. my csc becomes unknown while typing *#1234#
c. Both USB and UART options change to modem every phone boots
d. rooted and found csc folder is missing
e. wifi gpars work but Bluetooth can not be turned ON it becomes gray . sometimes phone hangs when I try to put it ON. it restarts automatically at times.
f. phone lags just like that of 4.3
is there any method settle problems of a., c., e. and f.??

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khaaba said:
I
Problems I found,
a. IMEI shows correctly with original SW version 01 but mobile shuts down suddenly quite frequently.
>>>Mobile as in what network or the phone ??
b. my csc becomes unknown while typing *#06#
>. Flash the correct CSC for your network.
d. rooted and found csc folder is missing
>. As above
Suggest wipe phone via recovery
Mounts and storage format sd card system cache data .
Boot to download mode and flash stock rom for your network .
Really you need to read and understand THE BASICS STICKIED FAQS AND GUIDES FIRST
Click to expand...
Click to collapse

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@JJEgan, thanks for the reply and tips!
a. IMEI shows correctly with original SW version 01 but mobile shuts down suddenly quite frequently.
>>>Mobile as in what network or the phone ??
---Mobile is in Vodafone India on my I9300
b. my csc becomes unknown while typing *#1234# [SOLVED]
>. Flash the correct CSC for your network.
--CSC folder has been recovered this way in the process: re-flashed 4.3 (swc) on existing 4.1.2 (xxemh4), sw version of imei changes to version 11 however IMEI remains same, them flashed another itv (4.1.2) as explained in another thread. I lost imei as it becomes 049....
then I flashed back to my country's 4.1.2 (xxemh4). got IMEIand its sw version correctly and my CSC folder also.
EVERYTIME I FLASH I DO FACTORY RESET AND CACHE WIPE compulsorily FROM stock recovery 3(e)
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I HAVE BEEN READING A LOT SINCE LAST FIVE DAYS, basics of flashing I guess I know as i have been doing this since 2009[Samsung Bada on GT-S6353]. I use only stock recovery and stock roms. my problems now reduce to:
1. Bluetooth non functional- can not make it ON. When I slide Bluetooth button to make it ON, phone either hangs and restarts automatically or Bluetooth button goes back original OFF position
2. phone's utilities reset to modem automatically [both UART & USB]. I check this when my s3 can not connect to pc or kies to reload my contacts on s3. This is checked by typing *#7284# and I need to set it UART => MODEM & USB => PDA. But these two setting reset to MODEM automatically whenever I check at later stage [when i found pc doesn't recognise s3].
3. phone lags just like 4.3 even in 4.1.2 [xxemh4]
I will do flashing again! Thanks once again!
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Flashing done! It doesn't help me

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Khaaba,
When your problem solved (Bluetooth, phone lags and so on) please do post here.
I am also facing same issues.
Thanks and regards

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Problem Solved
jit1108 said:
Khaaba,
When your problem solved (Bluetooth, phone lags and so on) please do post here.
I am also facing same issues.
Thanks and regards
Click to expand...
Click to collapse
Use this link and flash with "I9300XXEMG4_KOR_MULTI_FACTORY". That's the method I followed!
Everything is normal. Then I flashed my country's FW using Odin in PDA mode only!