I being using the forum (since my Xperia arc) and its being great (but I have to be honest, never registered here)
I got a good opportunity to buy a Xperia Z1 really cheap, because the original owner didnt know which version he has (he said C6903 but it was C6902 thanks for the guy I am going to show you the guide) So, he end up in a boot loop. Multiple user here reported Xperia z1 dead, but this happened because when the phone enters in a boot loop, the screen turns on without knowing it is on, so you never did the reset trick (Power+ Volume +) to shutdown the phone properly (or the red buttom in the SIM tray)
Also something really important happened, when I flashed the firmware C6903 into my C6902, this end up in a boot loop and most of the users are reporting, NO IMEI or BASEBAND (was saying unknown)
The user from the Dev here (Nidhal), helped me out to fix the phone, and thanks to that user on Ebay, I can clearly say that 90% of the brick is the user fault.
PLEASE REMEMBER, THIS IS ONLY FOR LOCKED BOOTLOADERS, DONT REPLY IF YOU HAVE UNLOCKED BOOTLOADER:
So lets start:
1- First, lets know what is your real version of the Z1 you have , in your SIM tray (take a look of attachment picture), simply detach the cover of the micro SIM card slot. Place a fingernail under the right or left edge of the label tray, then drag the tray outwards to view your model and IMEI (also SI), this will help you of the 80% of the problem.
2- Now that you really know which version you have (in my case is C6902), you can start downloading the correct firmware. IF YOU ARE IN 4.3, use this guide to downgrade to 4.2.2 (using NUTT firmware .534)
If you are in 4.2.2, just download the correct firmware from the label you got from your SIM slot.
3-Always remember to make a backup before flashing. When you are in flashtool, flash the firmware of your phone version and very important WIPE APPSLOG, CACHE and DATA.
Believe me, theres a lot of users think they have a model, but they aren't, so I hope you can fix your phone, like me.
Regards and good luck.