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How To fix Sony Xperia Lt18i Bootloop , Even after flash 

Learn How to flash sony xperia from here

To unbrick lt18i which you have flash with original firmware and still not booting ..

What you need to do is to flash a firmware called IS11S or SO-02C DOCOMO

Requirement:

Androxyde Sony FlashTool 

PC

USB cable 

lt18i Firmware Download one of the above 

Steps:

Open FlashTool utility from the Windows "Start" menu.

Wait for the application to sync devices from Github.

Once that is over, keep the Back button on the phone pressed, and connect the USB data cable. The indicator LED, near the micro USB port on the phone, will show a green color.

Click on the Flash button (the one with a lightning symbol in the toolbar of the application).

In the Bootmode Chooser dialog box that comes up, select "Flashmode" and click OK.

In the Firmware Selector window that opens up, against Source Folder, select the folder where you have downloaded the firmware file to.

Now click the Flash button.

If they ask you to disconnect and reconnect the cable, do as we did in the above steps, and following the instructions shown.

Now when the log window shows "INFO - Flashing Finished.", and "INFO - Please unplug and start your phone", disconnect the USB cable, and switch on the mobile phone.

That should do it. You should now see a fresh and working OS booting up

HOW TO UPGRADE,FLASH/UNBRICK HUAWEI DEVICES

 it is recommended to use Sandisk, Kingstone, or Kingmax, size is recommended that more than 1G.
Step Two:
Formatting SD card (optional).
Step three:
copy a large package path UPDATA.APP to the SD card > dload >UPDATA.APP.
Step four:
Check the root directory of the SD card you dload UPDATA.APP.
Step 5:
SD card into the phone is switched on in the main screen, press the Menu key, the system updates -&  SD card upgrade -> confirm -> upgrade, backup data, select the backup data; upgrade.
Step Six:
The upgrade process progress bar shows after the finish of the pending progress bar, the phone automatically restart.

METHOD 2

Step one:
Prepare an SD card,
Step Four:
Power Off phone, insert the SD card, while pressing the volume up button + volume down key switch button, the phone automatically enters the upgrade mode, and then begin the upgrade in Figure

Step Five: After the progress bar to go full fixed, the phone will automatically restart If you do not manually remove the battery.


METHOD 3

SP FLAS TOOL
Learn how to use it here

HOW TO FLASH / UNBRICK OR UPGRADE HTC PHONE WITH RUU FILES 

How to flash/unbrick HTC device if you messup either by rooting, installing some custom rom etc or you just want to get back to the default stock rom, follow the simple guide below.

Boot up your phone by pressing volume down and power button