Installing custom recovery, custom ROM, custom kernel and enabling root access on your Zenfone 2 are fine, but if you have not followed the instruction carefully, you might end up with a non-booting system. This situation is called soft bricked, which is hard to recover as it damages the bootloader.
Because of damaged bootloader, you will not be able to use ADB and fastboot command to restore the stock ROM on your Zenfone 2. However, you do not need to worry. You can unbrick a bricked Zenfone 2 (ZE550ML/ZE551ML) with a tool called xFSTK and restore stock ROM using ASUS FlashTool.
The xFSTK tool is a windows application that lets you restore damaged bootloader on your Zenfone 2 (ZE550ML/ZE551ML). After restoring bootloader, you can use Asus Flashtool to install the stock ROM.
Thanks to XDA user Sumit007s, in this guide, we will explain how to unbrick a bricked Zenfone 2 (ZE550ML/ZE551ML) by restoring lost bootloader on it. The below method requires little technical expertise. Read the guide carefully before proceeding.
How to Restore Bootloader on Zenfone 2 (ZE550ML/ZE551ML)
Disclaimer: Follow the instruction at your own risk. Ensure that your device is soft bricked before proceeding. We will not be liable if any damage occurs to your device and/or its components. It is recommended for technically talented users only.
2. Download xFSTK Downloader (v1.7) [HERE] and install it on your PC. This tool is used to restore bootloader into the memory chip.
4. Open xFSTK Downloader tool and select ‘MRD tab A0 / B0 + A0 + CRC….’ tab.
5. Click ‘Options’ and then click ‘Modify Settings’. Enable the box ‘GP Flag Override’ and set the value to 0x800000807. Click OK to proceed.
6. Click FWI DnX box and add the file dnx_fwr.bin (you have unzipped in step 3) Then, click IFWI and add the file ifwi.bin. Then click OS Image and add the bootloader file (droidboot_dnx.img.POS_sign.bin). After adding three files click ‘Begin download’ button.
[If your Zenfone restart, long press the power key to shut it down and then click power key to begin download]
7. When the process is completed, you will see ‘Success: FW+OS download complete’ message in the tool and your phone will boot into fastboot mode.