If your Asus ZenFone 2 ZE551ML is stuck in a bootloop, running a custom ROM you want to drop, or just lagging after years of use, flashing the stock firmware is the way to set it right. This guide has the official firmware download links and two ways to flash them, updated for 2026.
The ZenFone 2 ZE551ML (codename Z00A) shipped with Android 5.0 Lollipop and got its last official update to Android 6.0 Marshmallow, so that’s as far as the stock firmware goes.
Asus ZenFone 2 ZE551ML specifications
The ZenFone 2 ZE551ML has a 5.5-inch 1080 x 1920 IPS LCD with Gorilla Glass 3. It runs on a quad-core Intel Atom chip (Z3580 on the 4GB model, Z3560 on the 2GB model) with PowerVR G6430 graphics, and came in 2GB/16GB, 2GB/32GB and 4GB/64GB versions with a microSD slot. It has a 13MP rear camera with dual-LED flash, a 5MP front camera, and a 3000mAh battery with fast charging on the 4GB model.
Why flash stock firmware?
- Unbrick the phone or fix a bootloop
- Remove root, a custom ROM or custom recovery
- Go back to stock before selling the phone
- Fix lag, crashes or failed OTA updates
Download ZenFone 2 ZE551ML firmware
The links below come straight from Asus’s own download servers, so they’re the safest source. Flash the Marshmallow build unless you have a specific reason to go back to Lollipop, since it’s the final official version.
| Build |
Android |
Source |
Download |
| WW-4.21.40.352 (latest) |
6.0 Marshmallow |
Asus |
Download |
| WW-2.20.40.206 |
5.0 Lollipop |
Asus |
Download |
These are for the WW (Worldwide) variant, which suits most regions. If your phone is on a different carrier or country build (CN, TW, and so on), check the ZenFone 2 section on the Asus support site for the matching file, since flashing a different region’s build can cause problems.
You may see this firmware mirrored on sites like MediaFire. The Asus links above are the originals, so use them when you can and treat the mirrors as a fallback only.
Take a full backup first. Flashing firmware can erase your data, and if it’s interrupted the phone may not boot. Use firmware for the ZE551ML (Z00A) only, not the ZenFone 2 Laser or another model. GetDroidTips is not responsible for any damage to your device.
Before you start
Method 1: Install from the SD card (easiest)
This works on a normal, locked phone and doesn’t need a PC.
- Download the firmware zip and copy it to the root of your internal storage or a microSD card. Don’t rename it; the stock updater expects the original file name.
- Turn the phone off.
- Hold Volume Up + Power until the phone vibrates, then release to boot into recovery.
- Use the volume keys to select apply update from SD card (or from internal storage) and the power key to confirm.
- Select the firmware zip and wait for it to install.
- Choose reboot system now when it finishes.
Method 2: Install with ADB sideload
Use this if the SD card method won’t pick up the file.
- Copy the firmware zip into your ADB and Fastboot folder on the PC.
- Turn the phone off, then hold Volume Up + Power to boot into recovery.
- Select apply update from ADB with the volume and power keys.
- Connect the phone to the PC with a USB cable.
- Open a command window in the ADB folder and check the connection:
adb devices
If your phone’s serial number shows up, start the install, replacing the file name with your firmware’s exact name:
adb sideload UL-Z00A-WW-4.21.40.352-user.zip
Wait for it to reach 100% and finish, then reboot the phone from the recovery menu. The first boot can take a few minutes.
Frequently asked questions
What is the latest firmware for the ZenFone 2 ZE551ML?
Android 6.0 Marshmallow, build WW-4.21.40.352. Asus released no Android version after Marshmallow for this phone.
Do I need to unlock the bootloader to flash firmware?
No. Official firmware installs through the stock recovery on a locked bootloader, which is what makes it useful for unbricking.
Can I use this to remove a custom ROM?
Yes. Flashing the full stock firmware replaces a custom ROM and returns the phone to Asus’s software.
Can I get a newer Android version?
Only through a custom ROM. Our guide to install Android 7.1.2 Nougat on the ZenFone 2 covers that.
The firmware won’t flash. What should I try?
Make sure you downloaded the WW build for the Z00A and kept its original file name, and that the phone has enough charge. If the SD card method fails, use the ADB sideload method instead.
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