I recently tried to install Cyanogenmod 12.1 on my LG Optimus Pro (I bought an unlocked one on Amazon a while ago... it's an AT&T phone, but works fine with my T-Mobile SIM card).
I made a lot of mistakes over the course of a week, so this is one of those here's-what-finally-worked posts for my own future reference, as well as anyone else in the same boat.
Typical disclaimer: I'm not at fault if this stuff ruins your device. Flashing phones is not for the faint of heart.
One cool thing about this process, though, is that in theory you should be able to do all of these steps right from your phone (I took some detours that necessitated a computer). So unless I say otherwise, do all of this stuff right on your device:
This was a little trickier than usual:
Download and install the towelroot APK file. You will need to enable installing apps from untrusted sources---the phone will take you to the right place in the settings to turn this on if you just try to install it.
AT&T is pretty obnoxious with this one; you can't actually boot into recovery unless you first unlock the bootloader.
Open BusyBox, and click Install
This last step caused me all kinds of pain. The first time I installed it, none of the buttons would work. I never quite figured out an easy solution for this, and I wound up doing something silly (see the Detour section below). In the long run, I reset the phone to its stock condition, and then all of a sudden FreeGee worked just fine. If you find yourself in the same scenario, hopefully you can find a less drastic solution. But flashing the stock ROM should do it if nothing else does.
I actually had some problems with the latest snapshot (as of this post, it was the 2015-10-07 one): it couldn't recognize my SIM card. This may have something to do with me being on T-Mobile, and the phone was designed for AT&T. But I haven't had any problems with the nightly, so I'd recommend going with that if there isn't a newer snapshot.
Download the latest nightly build here. The one I used was added 2016-03-02.
Download whatever set of Google Apps you prefer from OpenGApps. As this guide is for CM 12.1 on the e980, you'll want the ARM Platform, and apps geared for 5.1. I went with the nano version, but I think any of them should be fine.
The e980 is a little tricky to get into recovery, even if you've set it up correctly. Restart the phone, and hold the volume up button as it starts up; hold the power button as well as soon as you see the LG screen. I think in theory you're supposed to hold the power button with the volume up button from the beginning, but for that seemed to force the phone to keep boot cycling.
Do a factory reset
Flash the CM 12.1 zip that you downloaded
Flash the GApps zip as well
Reboot the phone
There's a bug in CM 12.1 (again, as of this post) that breaks audio in phone calls. Before you bother installing this, though, it would be a good idea to test and see if you can call someone. If you're a T-Mobile customer and you can't hear anything, you'll want to download the patch zip file here. You'll probably need to do something different if you're not on T-Mobile, though, but this is where my helpfulness ends (I ran across different solutions for different carriers as I googled around).
You'll also need to change some settings on the phone before you reboot into recovery again. Download BuildProp Editor, and make sure your settings match these:
persist.audio.handset.mic.type=digital persist.audio.dualmic.config=true persist.audio.fluence.voicecall=true persist.audio.handset.mic=dmic persist.audio.fluence.mode=endfire persist.audio.lowlatency.rec=false
com.android.phonecrashing may have something to do with the APN settings getting some extra cruft somehow (see this post). To fix it, clear the Password and Server fields (should be asterisk(s)), and set MMS Port to 80.
When I encountered problems with FreeGee, and couldn't find anything that seemed relevant online, so I started trying silly things. Initially, I think I found an app on the app store that successfully installed an older version of the ClockworkMod recovery that was able to flash CyanogenMod, but all the Google Apps kept crashing. After reading that it might be the old ClockworkMod's fault, I tried to install TWRP using some command line
dd voodoo in the terminal emulator.
Bad mistake. It totally soft bricked the device and I couldn't even get into recovery anymore. To fix it, I had to do the following steps in Windows (I tried a bunch of things in OS X, but it turned out I needed the Windows drivers for the phone... and I don't think they make them for OS X, at least not for these kinds of operations).
Download and extract this stuff on your computer (you'll need 7-zip or something similar for the .rar file):
The TOT file
The DLL file
Once everything is extracted:
Install the LG Drivers
Install LGFlashTool, but don't launch it yet
Move Megalock.dll into
C:\LG\LGFlashTool (replace the existing one)
Launch LGFlashTool, choose manual mode, and select the TOT and DLL files
Click the yellow arrow button; when you see the "READY!" message, hold both the up and down arrow buttons on the phone, and plug it into the computer. It should go into download mode and all the original AT&T bloatware should be on its way.
If things don't get started, it took forever for me to figure out that LGFlashTool expects the phone to be on a certain port. With the drivers installed, your phone should at least show up in My Computer when it's plugged in. To set the port (I think COM41 is the default that LGFlashTool is looking for), you may need to find your phone in the Device Manager, right-click to show its Properties, and then go to Port Settings -> Advanced, and change the COM Port Number appropriately.
Hopefully some of this is useful. Happy modding!