Unlock Bootloader

Unlock Bootloader

visit Bootloader Unlock Sony and follow guide

1. Dial *#*#7378423#*#* this will bring you to the service menu
2. • Service > Service info > Configuration >
     Rooting status: unlock allowed must be YES
     if it says NO you cannot root your phone (no solution yet at this time)
   • Service > Service info > Configuration >
     write down your IMEI number to get your Unlock Code
3. Go to [unlockbootloader.sonymobile.com Bootloader Unlock Sony]
   read it and click through all pages to create your unlock request.
   If your IMEI doesn't fit, remove the last digit.
   
4. download & install the Android SDK
   
5. download Fastboot driver, but do not yet install it
   
6. take out usb cable & turn off your Xperia U
   
7. Enter FastBoot mode > hold the Volume Up button while putting back the USB cable. The Left Blue Led will turn on.
   
8. Install the fastboot driver while the LED is on
   
9. if you have problems to install the driver and the blue light turns off multiple times before you can update the driver: Open your device manager and go back to step 6
   At the moment you connect the USB cable, you’ll see an unknown device being highlighted. Right click on it to manually update the driver via Properties > Driver > Update Driver > Browse my computer and select the fastboot driver.
   
10. Windows start menu > typ CMD in search box
    
11. type CD C:\Program Files (x86)\android-sdk\platform-tools
    
12. run fastboot.exe -i 0x0fce getvar version
    verify response ( version: 0.x finished)
    if it says < waiting for device > go to back to step 6
    
    

If you continue you will not be able to auto-update your software again.
New updates need to be installed manually and can be found at XDA
Your warranty will most likely be voided. For more information read here
Last Step:
run fastboot.exe -i 0x0fce oem unlock 0x22222222
replace 22222222 with your unlock code.
done! reinstall your phone