What is Fastboot?

Fastboot allows you to re-flash partitions on your Android device. It works through PC command line and connects over a USB connection with your device. Not every Android device manufacturer allows direct Fastboot access to end-users, and unfortunately LG is one of them. But there’s a workaround for the LG G3..

The LG G3 download mode hides the fastboot mode behind it, so if we remove the download mode (laf) from the device we get direct access to fastboot on G3. Follow the instructions below to get started:

STEP 1: Remove LG G3 Download mode (laf)

Important! You’ll need root access to remove (laf), so make sure you’ve rooted your LG G3 before proceeding with the steps below:

→ Root LG G3 easily with PurpleDrake rOOT
  1. Download ADB files (download link →)
  2. Prepare your phone
    • Enable developer options: Go to your phone’s Settings » select About phone » Scroll to the bottom and tap on “Build number” seven times to enable developer options
    • Enable USB Debugging: Open phone’s Settings » select Developer options » Tick the “USB debugging” checkbox (under Debugging section)
  3. Connect your phone to PC with a USB cable and if/when a pop-up screen shows up on Phone asking you to ‘Allow USB debugging?‘ for the computer, make sure you tick the checkbox and tap OK
    Allow USB Debugging
  4. Navigate to the folder where you have extracted the adb and fastboot recovery files and hold SHIFT key on your keyboard and RIGHT CLICK and select “Open command window here” (as shown in the below image).
    Open-command-window-inside-any-folder
  5. Now type/paste the following ADB commands to backup LG Download Mode (laf), so you can restore it later when necessary 
    adb shell
su
dd if=/dev/block/platform/msm_sdcc.1/by-name/laf of=/sdcard/laf.img
  6. Now run the following command to remove laf and reboot your phone 
    dd if=/dev/zero of=/dev/block/platform/msm_sdcc.1/by-name/laf
adb reboot
  7. Your phone will now reboot and you boot into Fastboot mode now.

How to Boot LG G3 Fastboot Mode

The usual adb reboot bootloader command doesn’t work for the G3, so we’ll be using the same process used to boot LG G3 in download mode.

  • Disconnect your phone from PC.
  • Power off your G3 and wait 4-5 seconds after lights go off.
  • Hold the Volume Up button on phone, and while holding it, connect the phone to PC using a USB cable

via reddit