[LG V40 Guides, News, & Discussion] laminarturbulent:
I gotta say thanks for being so dedicated to helping out LG V40 users!

I've got somewhat good news to report:

After buying another V40 (also AT&T 30e), I was able to get the bootloader unlocked but now it's stuck going to fastboot (AT&T V40, not V35 engineering). FIXED after writing most of the post! Read to the end!

Here's what I did to get there:

This time I made sure to reboot once after enabling OEM unlock (this may be critical, not sure).

I followed this guide, naively thinking I could easily root without unlocking the bootloader (unlocked bootloader is a prerequisite but is not mentioned in the video).

Basically I used QFIL to get boot_a, used the Magisk app to patch boot_a, then loaded the patched boot_a with QFIL.

This resulted in the phone going to secure start-up and I did not know the password. So I factory reset it.

Then the phone switched to the boot_b slot, and booted to Android.

I tried step 1 again, patching boot_b with Magisk and flashing. Boom, red triangle, "all slots unbootable".

Reverting boot_a and boot_b did nothing, at this point I think some protection from the locked bootloader was triggered.

Loaded V35eng to abl_a (didn't know for sure if it was on slot b at this point). Could not get into fastboot, so I reverted abl_a. Loaded V35eng to boot_b, and was able to get into V35 engineering fastboot.

"fastboot oem unlock" worked this time, and the output was:

(bootloader) Erasing userdata and cache
OKAY [ 0.898s]
finished. total time: 0.899s

Which was different from my first V40, which always failed the first time trying fastboot oem unlock then said something like "already unlocked" the second time.

Reverted abl_b, and now the phone goes straight to V40 fastboot (at least it says it's unlocked!!!).

Reverting boot, abl, laf does not change this. USB still works which is fantastic, I can access EDL/9008 mode.

After updating fastboot.exe (I was using a version too old to have --set-active), fastboot --set-active=a worked and I was able to boot into Android on the stock boot_a.

I then loaded the Magisk-patched boot_a with QFIL, rebooted to Android, and got root!

So in the end I mostly followed the instructions on the first post to get it working. Lessons:

Definitely reboot at least once after choosing to "Enable OEM unlock" in developer settings.

Back up boot_a, boot_b, abl_a, abl_b before doing any flashing (you can also back up laf_a and laf_b but I didn't touch them this time).

Use a USB 2.0 A-to-C cable in a USB 2.0 port; ideally choose a port on the back of your motherboard if using a desktop.

I've noticed that USB 2.0 cables are generally more reliable but I can't say for certain that my use of a USB 3.1 type-C to type-C cable was what killed the USB data lines on my first V40 (at least I'm 90% sure they're dead, I haven't done an extensive clean of the port yet).

Get the latest fastboot.exe from this package.

Three-minute YouTube root tutorials are not to be trusted without reading many of the comments first.

EDIT 4/11/2022: Just finished rooting the first V40 since I replaced the USB port. Found my old post had a typo where I said I flashed V35eng to "boot_b" when I meant "abl_b" since the phone went to slot b which meant I couldn't access the V35 fastboot until flashing it to abl_b.