Is your boot-loader unlocked via Sony unlocking tool?
Caveat Emptor You will lose data proceeding below!
What phones can be unlocked?
To check if you phone can be unlocked, follow the steps below:
- First, you should confirm that it is possible to unlock the boot loader of your phone by checking the service menu. In your phone, open
the dialler and enter ##7378423## to access the service menu.
- In your phone, tap Service info > Configuration > Rooting Status. 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.
If the above says it cannot be unlocked, then you cannot unlock it and flash a custom ROM, you're out of luck! Follow the answer below by another poster who is 100% correct in usage of SUS (see below further down in my answer) to reset it.
If it says it can, then go ahead and follow the instructions to unlock the boot-loader. You will lose your settings.
Then go to XDA and install LegacyXperia which has the custom ClockWorkMod Recovery which can be activated by toggling the volume buttons upon start-up.
The quirk about the SEMC 2011 line of handsets is - there's no actual recovery partition, the custom boot image found within LegacyXperia handles the necessary loading of the recovery if the volume keys are pressed, and if not, continues loading Android as usual.
Oh and by the way, you might notice on some websites, where it governs using
fastboot, it will say something like this:
fastboot flash boot boot.img - that is incorrect!
You must specify
fastboot -i 0xfce flash boot boot.img, its the
-i 0xfce part that is omitted. It will not work without it.
If you wish to use a more powerful tool unlike SUS tool (Sony companion tool that checks for firmware updates), you can use this tool called Flashtool which can do a lot more than the SUS software itself, such as re-locking the boot-loader (if reverting back to stock).
The above is for the more adventurous kind of person, if not the other answer by the poster is more suited.