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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