Install the "Flashmode" and "Fastboot" drivers usually found in the drivers folder of the Flashtool installation directory.
Download the 02G 317 R12C remove lock file from a trusted firmware repository or community forum. Download 02G 317 R12C Remove Lock ftf
The Flashtool log should indicate that the device is connected in and the flashing process will begin. Install the "Flashmode" and "Fastboot" drivers usually found
Click the button in Flashtool. The software will prepare the files and then prompt you to connect your phone. Turn off your Sony Xperia device completely. Press and hold the Volume Down button. Download 02G 317 R12C Remove Lock ftf