Root, Bootloader Unlocking & Relocking, Firmware

Download:
http://j.mp/xps_fastboot_pkg [fastboot package with usb drivers]

1. Confirm that it is possible to unlock the boot loader

1. Open the dialler and enter *#*#7378423#*#* to access the service menu.
2. Service info > Configuration > Rooting Status
3. If Bootloader unlock allowed says Yes, then you can continue with the next step. If it says No, or if the status is missing, your device cannot be unlocked.

2. Unlocking Bootloader

1. Get the IMEI code for ur device: type *#06# in phone dialer.
2. Go to: http://unlockbootloader.sonymobile.com/ .
3. Scroll to the bottom of the page and click "Continue". Finally you will get a "Create Unlock request" page, enter your IMEI (last digit needs to be removed), name and email address. You should now receive your unique unlock key, its displayed on screen, SAVE THIS KEY SAFELY.
4. Extract fastboot_with_Android_USB_file.rar.
5. Power off your phone and keeping the Volume Up button pressed connect the USB cable to PC this will trigger FASTBOOT mode(Blue) and windows will start driver installation,and point the driver install to that location (usb_driver) folder in fastboot folder.
6. Hold SHIFT and right click on the background of the folder fastboot Select 'Open command window here'. In the cmd windows type fastboot.exe -i 0x0fce getvar version . If a value is returned 0.5, it means the phone is correctly connected.
7. Type fastboot.exe -i 0x0fce oem unlock 0xKEY  and replace KEY  with the key which u saved earlier.
8. Your device should now be bootloader unlocked.

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###[Relock Bootloader]###
Download:
http://androxyde.github.com/Flashtool/ [FlashTool]
http://tiny.cc/Add-on_Device_ST25 [Flashtool Add-on Device ST25]
http://tiny.cc/ST25i_Relock_Bootloader [Relock Bootloader FTF]

Relocking Bootloader

1. Extract Flashtool & install \Flashtool\drivers\Flashtool-drivers.exe
2. Extract 'Flashtool Add-on Device ST25.rar' to folder Flashtool.
3. Put Xperia_U_ST25i_Relock_Bootloader.ftf to \Flashtool\firmwares
4. Run Flashtool, Click Flash(electric symbol). Click Flashmode then OK.
5. Choose version --> Relock Bootloader  then OK.
6. Power off your phone and keeping the Volume Down button pressed connect the USB cable to PC this will trigger Flashmode (Green).
7. Your device should now be bootloader locked.

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###[ROOT]###
Download:
http://j.mp/xps_fastboot_pkg [fastboot package with usb drivers]
http://tiny.cc/ST25i_Pre-Rooted_IMG [Pre-Rooted system.img]

Note: To do this, you phone must unlocked bootloader

Rooting

1. Extract fastboot_with_Android_USB_file.rar .
2. Extract Sony_Xperia_U_ST25i_v2.3.7_Pre_Rooted.rar to \fastboot\Sony_Xperia_U_ST25i_v2.3.7_Pre_Rooted.img
3. Power off your phone and keeping the Volume Up button pressed connect the USB cable to PC this will trigger FASTBOOT mode(Blue).
4. Hold SHIFT and right click on the background of the folder fastboot Select 'Open command window here'. In the cmd windows type fastboot flash system Sony_Xperia_U_ST25i_v2.3.7_Pre_Rooted.img
5. Power ON your device
6. Your device should now be ROOTED.

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###[Flash original .FTF Firmware]###
Download:
http://androxyde.github.com/Flashtool/ [FlashTool]
http://tiny.cc/Add-on_Device_ST25 [Flashtool Add-on Device ST25]
http://tiny.cc/ST25_6-0-B-1-564_FTF [Firmware 6.0.B.1.564]

Flashing original .FTF Firmware

1. Extract Flashtool & install \Flashtool\drivers\Flashtool-drivers.exe
2. Extract 'Xperia_U_ST25i_v2.3.7(6.0.B.1.564)_Generic.rar' to folder Flashtool.
3. Put Xperia_U_ST25i_v2.3.7(6.0.B.1.564)_Generic.ftf to \Flashtool\firmwares
4. Run Flashtool, Click Flash(electric symbol). Click Flashmode then OK.
5. Choose version --> v2.3.7  then OK.
6. Power off your phone and keeping the Volume Down button pressed connect the USB cable to PC this will trigger Flashmode (Green).
7. Your device should now be Original Firmware.

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###[Download]###
[fastboot package with usb drivers] http://j.mp/xps_fastboot_pkg
[FlashTool] http://androxyde.github.com/Flashtool/
[Flashtool Add-on Device ST25] http://tiny.cc/Add-on_Device_ST25
[Relock Bootloader FTF] http://tiny.cc/ST25i_Relock_Bootloader
[Pre-Rooted system.img] http://tiny.cc/ST25i_Pre-Rooted_IMG
[Firmware 6.0.B.1.564] http://tiny.cc/ST25_6-0-B-1-564_FTF
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###[Refer]###
[Bootloader Unlocking & Relocking] http://forum.xda-developers.com/showthread.php?t=1527159
[Relock bootloader!] http://forum.xda-developers.com/showthread.php?t=1134335
[Unpacking and repacking stock rom .img files] http://forum.xda-developers.com