[Xperia Arc Q&A, Help & Troubleshooting] deejay315:
Hey, yesterday I managed to make my Arc S unbootable by installing custom roms, flashing boot.img's, etc, and eventually got it to the stage of not turning on atall, and when pressing power button, the LED just flashed green for less than a second.

This guide will take you through reinstalling the Stock OS on your phone, and relocking the bootloader, so your device is just as it was when you bought it.

Please note: I will not take responsibility for any damages this may cause. This worked for me, and should work for you, but I cannot guarantee anything.

So first of all, your bootloader must be unlocked for this tutorial to work, but if you have been flashing custom ROM's, you will have had to unlock it anyway.

The supported devices for this tutorial as below, and also, please note the key you will need to press to enter flash mode:

Xperia arc (LT15i) – back

Xperia arc S (LT18i) – back

Xperia Tablet Z (SGP321) – volume down

Xperia go (ST27i) – back

Xperia neo V (MT11i) – volume down

Xperia ray (ST18i) – volume down

Xperia P (LT22i) – volume down

Xperia ZL (C6503) – volume down

Xperia Z (C6603) – volume down

Xperia PLAY (R800i) – back

Xperia E dual (C1605)– volume down

Xperia E (C1505) – volume down

Xperia T (LT30p) – volume down

Xperia S (LT26i) – volume down

Next, you must download the Sony Flashtool (Emma) from this link, which we will be using to get your firmware back to stock.

So when you have got this downloaded, install it using the .exe file, then goto the folder that it is installed, and paste the 'Customization.ini', that is also in the zip, in there.

The installation folder is usually

“C:\Program Files (x86)\Sony Mobile\Emma\” for 64 bit Windows or

“C:\Program Files\Sony Mobile\Emma\” for 32 bit Windows

Now run the program called Emma, which will be under the 'Sony Mobile' folder in your start menu (Or at the end of your Start screen on Windows 8), you should get an interface like this:

Now, make sure your phone is off, and hold down the button on your phone that is next to your phone name on the list at the top of the post, whilst plugging in the USB cable, and stop pressing it when your phones LED turns green:

If the program shows a message like this:

You cannot continue with this tutorial, as your bootloader is locked.

PLEASE NOTE:

The next step WILL wipe your phone

If your bootloader is unlocked, you should get a list of OS's on the right hand side, select one, and click 'Apply Service'.

Now just leave your phone and computer and go and get a cup of coffee while it completes!

When it is done, you can unplug your phone, then turn it on, and you should have stock Android again!

Next we will relock the Bootloader (Note: This only works on Arc/Arc S, so do not attempt on any other device), so download this file (A repacked version of X10Flasher, with the .ftf file already there), and extract it to your computer (I know its called X10Flasher, but it works fine on Arc/Arc S, like in this video).

(Thanks to Bin4ry for making this tool!)

When you have done this, open the X10FlashTool.exe file, and click Flash in the top left corner, then click 'Ok'.

Now, as we did earlier, turn off your phone, and connect it to your computer whilst holding the Flash key.

Now wait for the program to do its magic, and unplug your phone, and it will be as good as new!

You can now use the Sony Update Service to update your phone to the newest version!

Thank you for reading!