- Fully charge the phone
- Fully backup the phone ( guide in here)
- Enable USB debugging (Settings>Developer options)
This is not a risky process, if anything goes wrong, your device can be bricked. Applying this method at your own risk. I can’t claim the damages. This method only for the Xperia S and U devices.
- Install the downloaded flashtool and run the application.
- Copy your device downloaded relock bootloader “.ftf” files, into the “C:\Flashtool\firmwares” folder.
- Power off the phone and wait for a few seconds.
- Press and hold the “Volume Down” key and same time connect the phone to the computer. If Windows starts install fastboot drivers, wait for a few seconds to complete the installation. Now you can see your phone LED turn to green.
- If flashtool display “Device connected in flash mod”, you can go to the next step. If it fail to identify your phone install the drivers and follow the process again.
- Click Flashtool “Thunder” symbol contains icon.
- In the “Mode selector” window tick “Flashmode”
- Firmware selection windows, choose the correct relock bootloader firmware
- Finally click “OK” button, now it starts the installation process.
- When the relock bootloader ftf file installation complete, flash stock firmware using flashtool.
- After all the installation complete, disconnect the phone from the computer and power on.
- Now you can verify your bootloader locked. Go to dial pad and type *#*#7378423#*#*
- Navigate to “Service>Configuration” if it displays “Bootloader unlock allowed:Yes”. You relock the bootloader.
Now install stock ROM using PC companion, Sony update service.