[G3 Android Development] pol098:
I have soft-bricked an LG G3, and would be very grateful for any advice, please. (My background is in software, but my Unix is both out-of-date and very rusty, and I'm not into phones at all. So I'm happy with files, file systems, and binary editing if necessary, but otherwise at sea regarding, e.g., have I deleted the kernel and do I need to attend to that first).

LG G3 D855 16GB/2GB, rooted and Fulmics 6.9 was installed when I bought it, I installed TWRP (v3.1.1.0, which seems to be a mistake). I have an old (2017) nandroid backup, and a new one made AFTER the problem arose.

I was having the "screen stays dark after a phone call" problem, so went into the hidden menu and carefully and correctly went into the bit related to calibrating the motion sensor. The phone shut down and would then no longer boot, with "[670] boot certification verify / secure booting error! / cause: boot certification verify" with the indicator light flashing blue and red, an issue that others have described. I can start up the twerp recovery.

My plan was to restore the working 2017 backup with twerp, then, if possible, use Titanium Backup to restore applications and their data from the post-brick backup. I don't have any important data, but I'd like to get them back, including applications which are no longer available; but at least I'd like my phone back, even if "factory reset".

After restoring the 2017 backup (apparently successfully) the phone remained bricked in the same way. I wiped everything (including ultimately internal, but not external, storage - bye bye); still bricked. I then decided to update to Fulmics 8.0 (latest), so copied the ZIP onto the root of the SD card, wiped again, and ran twerp INSTALL. Result "failed to map file /external_sd/FulmicsROM-8.0.zip". Searched; known problem, use older version of TWRP, easy. Except that I don't know if it's possible to install the older TRWRP on a phone with TWRP 3.1.1.0 installed, but no operating system. I haven't yet done anything with adb on a Windows computer; can adb do that? I don't remember if I had enabled USB debugging; if not I'm presumably stuffed (adb doesn't find any devices). (Or does adb require a working Android?)

So I'm stuck with a dead phone, which I'd like to revive and, if possible, restore data to using Titanium on the post-brick backup. One issue I'm unsure about: having deleted internal storage, should I be able to install Fulmics without first installing a kernel?

Looking through threads, I find people with the new-TWRP problem on other phones have worked round it by extracting boot.img from the ROM ZIP file, installing it, then installing the ROM as usual. The question that arise with Fulmics: there are two eligible boot files for the D855, d855.img and d855_lz4.img; obviously one using better LZ4 compression, but which do I need? I suppose it depends on the kernel?

So, summarising questions: with a working TWRP but no Android on a D855 LG G3

-Help! What can I do?

- Can I install an older TWRP on a phone with 3.1.1.0 installed but no Android?

- Do I need to install anything before the ROM?

- If it's not possible to downgrade TWRP, should I try d855.img or d855_lz4.img (or neither!) before flashing the ROM?

Apologies for the long post, but I didn't want to leave unanswered questions. Thanks for looking at this and best wishes.