Android: Easy Root Xperia Arc With A Locked Bootloader
Having a Sony Xperia phone with a locked bootloader makes flashing and rooting your phone a little more difficult and we have to find other ways to get around this. However, it is possible and this is what you want to do.
Get your Xperia Arc up to Android version 4.0.4, not 4.0.3, but 4.0.4. Important, you can do this by flashing it manually (Flashboot) or using Sony’s PC Companion to update it.
Now, you’re going to want to flash the firmware back to 4.1.B.0.431 (Ready To Root) with the following .ftf file. This is for the Arc LT15i, but if you look around the Internet you can find the right files for the Arc S LT18i.