[Sony Xperia T, TL, TX, V] yujinmac:
I also encountered this problem recently in my ____>>> Xperia TX <<<____..

Don't panic, your phone is still fine unless you did something else...

The problem is that you can't upgrade directly from 4.0.4 to 4.3 or 4.1 even you have to wipe the data and apps (in my case), I tried several 4.3 firmwares posted here ...

What Ii did are:

1. Find and download other stock ROM on the list of the same OS as you phone (4.0.4 in my case) but might be of different build version (mine for example is 7.0.A.105 then I get the 7.0.A.122 build or you can try the same build version but from different country - i don't know if the version build has the same to other country, I just thought). I also suggest you have to get from other countries (my phone is from middle east then I get Russia stock ROM because no update on my recent as stated by PC Companion so I stuck at 4.0.4).

2. Follow what Flashtool tutorial says just make sure at first that you installed :

a. Latest version of flashtool;

b. Drivers on the flashtool folder. You will get drivers failed to install in Windows10 64bit in my experience so I suggest you have to do this in Windows7. Others say you have to disable certificates authentication etc in Windows 8 (thinking the same case in Windows10) but I don't have time for it. You then check/tick the flashmode drivers, fastboot drivers and your phone model driver (mine is Xperia TX drivers)

c. PC companion latest version (you need to exit this before flashing in my case) - the reason is you can get the driver for your Xperia though you have it already in Sony Flashtool but just to double sure.

3. Once you downloaded the firmware, you can then try to flash your phone with the cable from Sony or better. if there's any

Note: Run the flash as Administrator in my case. Better also to totally disable your antivirus to avoid any conflict/errors if any (in my opinion) while flashing (though Sony flashtool is safe, and also the firmware or stock ROM from this thread (in my case)) .

4. Check/tick those in Wipe area and also exclude all in .ta extension.

NOTES:

You DON'T need to plug your phone at first. You have to plug your phone when the Flash requested to do so.

Make sure you also hit the power-volume down or up to completely unmount your phone from battery if non-removable, if removable you remove it first for 5-10 seconds BEFORE flashing.

While flashing you have to patiently press down the flash button (in my case volume down) until finish. I did NOT try to release the button once flashing started cause I want to fix my phone ASAP. This will take only less than 5mins.

5. Then hit flash.

6. Voila! You successfully flash your phone! By then you can unplug your phone once "finish" flashing.

7. Turn ON your phone and it should boot then.

It is good once you see the xperia logo (in my case) until started. If turns off by itself, try to turn ON again (in my case) and it should boot and run normally.

8. Remember: You have the same OS as before but then you can try the PC companion for the latest one and follow the instructions on updating. If none, and you insist for the latest, you'll have to find other stock ROM from other countries who have already had the latest update from PC companion. In my case, after flashing using Russia 4.0.4, then change language to English, then PC Companion shows that I can update to the latest Firmware (for Xperia TX) which is 4.3.

Note: If your storage is 16gb which is normally around 10gb space, after flashing I got 1.8gb.. Just run back-up & reset > "factory data reset" from settings then wipe internal (save all necessary stuffs first to your Desktop/Laptop/usb) and you will get a clean 10gb space internal storage. Your phone will stay at the same OS (mine flash to 4.0.4 then update to 4.3 then run factory reset, result: ~10gb space at 4.3 OS)

Sorry, this is for complete noob as me.

Hope these helps...