[G2 Q&A, Help & Troubleshooting] Stiesen:
Hi everyone

I am posting this because I fail to flash TWRP on my LG G2 (D802, 30c); I would like to flash LineageOS. I have read many guides and posts - however, none worked. So I turn to you for advice -- would you help me, please?

I have managed to flash Cyanogen to my Samsung S2 a few years ago and TWRP and LineageOS to my LG G5 (H850) last year -- the S2 was easy, the G5 very tricky, but the G2 kills me. The G5 took a real effort because no guide worked for my phone, and I had to take a little from this guide, a little from that and add a bit of my own (let me know if anyone is interested in how I flashed LOS to my G5). I have tried the same with my G2 -- but I seem to have come to a dead end.

I am writing all that that to tell you that I have gone to great length before troubling you. Here is in summary what I have done so far and where I stand:

I have installed the sdk toolkit, adb and fastboot properly when I flashed my G5 last year.

Now I have started with the instructions from the LineageOS wiki (wiki.lineageos.org/devices/d802/install). I have activated USB debugging and rooted my phone using One Click root (Before One Click Root for the G2 was available, I could not even root my phone).

Then I flashed TWRP using "dd" as explained in the installation guide in the LineagOS wiki. I also tried it by flashing through the TWRP app (the app said it was flashed successfully). I tried TWRP 3.2.3.0 and 2.8.7.0 for the D802. The result always was as set out below.

After that I turned off my phone and started I by holding power + Volume down, releasing power briefly after the LG logo appeared. I went through factory reset (selected yes and yes), and here the problems begin (the same when I use “adb reboot recovery”). The phone should start into TWRP, but it does not. Instead the start screen with the LG logo appears, and in the upper left corner the following message appears:

ERROR: boot certification verify

[26480]------------------------------------- -

[26480]

[26520] Secure booting Error!

[26570] Cause: boot certification verify

[26620]

[26670]-------------------------------------- -​

If the numbers in the square brackets make sense to you: They change every time I boot.

The next screen reads

------------------------------------------------- -

Critical Error!

------------------------------------------------- -

DemiGod Crash Handler : Critical Error!

Some information on how to reboot or access Download mode]

------------------------------------------------- -

Board Info Rev. : 1.1 speed.pvs : 1.3 serial : [...]

------------------------------------------------- -​

I have tried many things, including several resets using LG FLash Tool 2014 to flash an entirely clean stock rom and starting fresh rooting and flashing. I also tried LG G2 D802 One Click Root Recovery V1.1, which even failed to root the phone.

My feeling is that the phone prevents me from properly installing TWRP. It does not allow me to boot into the bootloader using "adb reboot bootloader" (the command just restarts the phone). Therefore, I cannot put it into fastboot mode (I also tried the volume-up button method to put it into fastboot and failed), which may be crucial to properly install TWRP. In this regard, it may be relevant that the Developer Options show no option "OEM unlock"; that might be necessary to access the bootloader and fastboot mode…

What do you think? Do you have an idea how to solve the problem?

Thanks a lot and cheers!