[Galaxy S III Q&A, Help & Troubleshooting] kinggimped:
OK, first of all I've read through pretty much every page of this thread and have tried so many things, and I'm still not having any success. If somebody could help me I would appreciate it HUGELY!

Currently running stock 4.3 (I9300OXAGMJ9_BTU) on my I9300, but hate stock ROMs and really want to get real Android back on there. Baseband is XXUGMK6, registers with network (China Unicom) fine.

I was running PA 4.2.1 for a while and decided to upgrade to a 4.3 build. Didn't realise that doing so would update my EFS partition to EFSv2, so didn't do an EFS backup (I know, I know... I figured it would be an easy update).

The 4.3 PA ROM didn't work (IMEI/baseband unknown), and after trying a few other modems to no joy I gave up and downloaded the stock 4.3, and have been using that the past few months. But I hate it. However, at least it connects to the network fine and doesn't give me baseband/IMEI unknown errors like I was getting after I updated to EFSv2. It has horrible battery drain, too (doesn't seem to be the battery since airplane mode doesn't drain, I'm guessing it's a modem/EFS problem).

So far I've tried flashing the latest CyanogenMod nightly for my phone, which is based on 4.4.2. Runs wonderfully. Baseband is still XXUGMK6, IMEI is intact and correct, but won't register with network. Tried Suleyman's patched modem file, tried using zuluman's patcher to modify a stock XXUGMK6 modem.bin, still won't register with network. Flashing any other modem gives me baseband/IMEI unknown.

After reading some of the advice over the last few pages of this thread I tried to install AOSB, also based on 4.4.2. Again, installed fine, baseband still XXUGMK6, IMEI intact and correct, still won't register with network.

Here's where it gets weird - with both these custom ROMs it DOES connect to the network for 1-2 seconds, but then immediately disconnects and says "no service". So I assume there must be something going wrong in the actual registration process. Another weird thing is that since flashing that CyanogenMod ROM my phone's serial number is now different from the one on the label on the back of my phone - it has changed to a 4df... serial, which I have read is a generic serial number. Could this be why it's failing to register on the network?

I reflashed a nandroid backup of my 4.3 stock ROM, baseband is still XXUGMK6, IMEI intact and correct, but serial number is the weird 4df one... and yet it registers with network fine! Can make calls, receive texts, everything.

So why, if all this information is exactly the same, will it only register on the network with the stock ROM? Am I missing something here? Could this be related to the OpenRIL used by CM-based ROMs? I've tried using the init.d script too, but no luck (it's possible I'm installing it wrongly or in the wrong sequence). Flashing the patched XXUGMK6 kernel results in the ROM being unable to get past the boot logo (I'm guessing CM-based ROMs are no compatible with this kernel).

I feel like I have tried every solution available in this thread, and nothing works. If somebody could help me I'd be really grateful.

Sorry for long post, just wanted to be as detailed as possible.