This might help someone having these no service/ no sim card no gps fix issues with the galaxy s3 i9300, that might have to do with efs folder change and IMEI corruption.
Okay so i solved these no service issues and ill share my experience so maybe someone will find it helpful.
So the issue has to do with the famous efs folder conundrum, though tbh im not a developer so i cant say exactly what was the issue or even be a 100% sure it was the efs folder.
So I got me a used malfunctioning i9300 which i then repaired at a lab and proceeded to install a new TW based rom, my first mistake was not backing up efs folder and creating a nandroid, but i havnt realized that yet. After flashing, I noticed that i have no gps, but i didnt know if the gps was working with original firmware because i neglected to check that or back it up, so i decided to put a pin in it for the time being and went on to check out some roms.
At first, I flashed couple of JB tw based roms, all was well (except for the gps), then i started to check out some KK roms, those didnt play well with my device, I got no service in every single rom i tried, You name it ive tried it, none fixed the service issue. Moreover i also tried a few JB AOSP based roms, among them the 4.3 jb pac-man rom, those had the same no service/no sim issues. Meanwhile whenever i un-root and flash with odin back to stock or restore a nandroid i made while on 4.3 foxhound rom (with service working and a supposedly good copy of the efs folder) service is restored. I would also mention that I had more backups of the efs from the working "foxhound" i made with different tools and when i restored these folders when on a rom with no service it didnt fix the issue.
Here are the solutions ive encountered and tried that didnt work:
-restoring efs with efs pro, samsung tool, efs backup -restore cmd tool.
-flashing latest firmware and flashing back to kk
-flashing all kinds of modems (those pre efs folder change (xxemg4) and those after)
-flashing latest twrp/cwm recovery
-trying to edit "nv_data.bin" with hex editor
-some more ways in trying to manipulate the efs folder.
-using samsung service codes on stock, which i found regarding this issue, except from the *#06# code, for imei, pretty much none worked for me.
Its important to mention that when on stock firmware or a JB TW based rom i would see my baseband version in settings and when I tried the *#06# code for checking my imei it gave me a serial number with many digits that I assumed was indeed my correct IMEI number, and when i was on kk roms or non tw JB roms i would get the same popup with no numbers and in settings i would see "unknown baseband" .
To add to that, whenever i rooted and installed a kk rom or a jb non tw rom and tried to restore efs with efs professional i would get a message the device is not perm rooted (but thats neither here nor there).
This research took me about 5-6 days and the end of which i was ready to give up on having a KK rom or a non TW rom for that matter, NOTHING seemed to fix the problem. I just had some fight in me for one more hopeless attempt. I read somewhere that a flashing of cm 10.1 and then going to settings -> more and ther change some network stuff from this to that would help, I said what the heck thats simple enough. I flashed CM 10.1.3 for i9300 and went into the settings but the options i read about werent the same so i did move one settings around in the more section, cant remember what it was but it was nothing i knew about anyway, then went on to recovery to flash carbon rom 4.4 and to my enormous surprise i had service and connection and to my astonishment i also got back the GPS as a bonus ! yay ! I bet you forgot about that too by now…
This ends on a weird note though, though everything is at working order now and the IMEI number i get is the same as the one i get when on stock rom it does not match the number on the back of the phone under the battary.
Hope this helps someone.