ThinkPad T420 BIOS Mod with downgrade support for 1.48+

Updated 2018-02-08. Written by sdx1, adapted from thinkwiki.org/wiki/Custom_BIOS. Originally published 2017-12-09.

The ThinkPad T420 and other xx20 laptops have long suffered from a crippling bug that causes 100% CPU usage when an eGPU and 16GB of RAM are installed simultaneously. This most likely stems from a lack of support in BIOS for 1600 and 1866 MHz memory modules. Lenovo has never bothered to release a patch, but Oleh and later BDMaster from BIOS-Mods released a custom BIOS fixing the issue. For ease of access, I've uploaded his BIOS here, along with a nicer BIOS logo and a method to downgrade from versions 1.48 and 1.49.

Features

• Based on v1.46
• WLAN whitelist removed - can use any wireless card
• 1600MHz & 1866MHz RAM support
• 16GB RAM Support, stable with an eGPU
• Lots of extra menus available for additional configuration
• Updated splash logo included
• Can downgrade from 1.48 or 1.49 by using winflash's /sv /sd options

Disclaimer

This software can and may turn your computer into an unusable brick. I didn't write this software, and I offer no support, guarantee, or warranty in relation to this product.

Downloads

t420-bios-sdx1.zip - For T420 and T420i ONLY, v1.46, 8.22MB

Installation

1. Plug in your ThinkPad. Make sure it's fully charged.
2. Unzip the archive.
3. Run "Downgrade from 1.48 or later.cmd". Follow the on-screen instructions.
4. (Optional--to get the new logo) After restarting, run WINUPTP.EXE from the same folder and follow instructions. Click yes when prompted about the startup image and the older version.