OK, here is what I did.
The phone: Samsung S4 i9505, firmware 4.4.2
Problem: IMEI = null/null and Baseband = unknown
Cannot get into any other 'service mode' menu, only *#0808# works.
(1) Make sure the phone is rooted and has SuperSU installed and also enable su during boot is ticked under SuperSU settings. (note: when you tap SuperSU it will ask you to disable 'KNOX', tap yes (It's to do with security)
(2) Make sure USB debugging is enabled (under settings menu on phone)
(3) Use "EFS_Professional_2.0.91_Release" to backup everything up (don't worry if you can't do it, but it's lot safer to mess about afterwards)
(4) Search google to find I9505 (or your mobiles model number) EFS backup folder/files/partations and save them onto your PC somewhere. I've been told they're also available on Z3X shell support files.
(5) Connect phone to PC (using normal charger cable) and load Odin307 > search for the EFS files you downloaded and flash by ticking "bootloader" (efs.tar.gz - efs.img - UPDATE_EFS_GT... files names)
(6) When it reboots it may show IMEI = 01/ or only afew numbers (thats good news)
(7) Connect your phone and your Z3X box and load laters Samsung tool.
(
Open "Device Manager" on your Windows PC and make sure all the relevant drivers are loaded for the Z3X and Samsung port drivers for your mobile.
(9) Type *#0808# and put phone in 'DM+MODEM+ADB' and reboot. [IMPORTANT: keep phone always connected to Z3X box]
(10) Once rebooted and back into Samsung tool > Choose Samsung port (try both to see which one works) - by the way the "read phone" doesn't work so don't use that as a guide to see if it connects or not) Each function on Samsung tool will only work when the phone is connect into certain mode.
(11) Use IMEI repair function and tick new security and re-set EFS > type the original IMEI (with is loacted under battery of phone and serial number in all CAPS, not lowercase)
(12) Does the usual and reboots and if it fails at "Bypass MSL" (which had happend to me) just keep retrying and rebooting (with 'DM+MODEM+ADB' and reboot) and after a few tries and it succeed.
(13) Once you've got the IMEI and Baseband back it will need to be Flashed again using Odin307 with the proper original 4.4.2 full kitkat, which had just been released a few days a go (google it). I have had the beta version on.
All done!
OK, try this modem file first:
Modem (CP_I9505XXUFNA5_373648_REV06_user_low_ship_MULTI_CERT.tar through Odin)
https://mega.co.nz/#!UxBUVZjJ!JPFZH3WuFGLzS3fp2lHd1TF9d5t4nnj1lgXh74Mldqg-----
if it doesnt work then this bootloader:
Bootloader (BL_I9505XXUFNA5_373648_REV06_user_low_ship_MULTI_CERT.tar through Odin)
https://mega.co.nz/#!ZhYiHRKI!UJ3m7yRFwFofoW6dg4I6E28pfH0oiHyjb6nPN8O6Qj0Keep an eye on this thread as I will be updating files locations to help you. Remeber to reboot and check if null IMEI changes to 01/ or something before going next stage.
Basically, what needs to be done before following the steps above is to flash (using Odin, select - "phone") an older version "modem.bin" file for your model, try several and then use Z3X to attempting unlock or IMEI repair. This step is very important if you have updated newer firmware such as 4.3 or 4.4.2, if you don't do this, you'll get security errors such as MSL error. These files can be found using the google search engine. So try again a few times before giving up.
If this method fails it is more likely a hardware fault.
Check here:
http://forum.xda-developers.com/showthread.php?t=2192025Any foreign one will do as it will be unlocked anyway.
Unfortunately, the most likely problem is a hardware fault on the motherboard. The baseband I.C chip will need to be reballed or replaced.
