STATUS_ASSERTION_FAILURE (0XC0000420) Fix: Driver Signing Blocks Assertion

This error is infuriating — I know

You're working, gaming, or just booting up, and bam — blue screen with STATUS_ASSERTION_FAILURE (0XC0000420). It's cryptic and feels like Windows is punishing you for no reason. But here's the thing: this error is almost always a driver that failed its own internal sanity check. And the fix is straightforward once you know where to look.

The quick fix: enable test signing mode

If you're dealing with a unsigned or self-signed driver, Windows will crash it on purpose if it fails an assertion. The fastest way around this is to enable test signing. This lets unsigned drivers load without the kernel throwing a fit.

1. Open Command Prompt as Administrator (press Win+X, then select "Command Prompt (Admin)" or "Terminal (Admin)").
2. Type this command and hit Enter:

bcdedit /set testsigning on

3. Reboot your machine.

You'll see a watermark in the bottom-right corner saying "Test Mode." That's normal. If the error goes away, you've found the culprit — a driver that's not properly signed or was corrupted.

To turn test signing off later, run:

bcdedit /set testsigning off

Why does this work?

Assertion failures are basically a driver screaming "I expected X, but got Y!" and then self-destructing. The kernel sees this as a fatal bugcheck. When you enable test signing, Windows relaxes its strict driver signing policy. This doesn't fix a buggy driver, but it stops the kernel from crashing the system just because the driver isn't signed. The driver still has to run — if it's a real bug, you'll get a different crash.

But here's a specific real-world trigger: I've seen this error pop up on Windows 10 22H2 and Windows 11 23H2 after installing older network drivers (like from a Realtek PCIe NIC that shipped with Windows 7-era software). The driver's assertion check fails because it tries to allocate memory in a way Windows 10 no longer allows. Test signing lets it load anyway.

Less common variations and deeper fixes

Test signing doesn't always work. Here's what else to try.

Option 1: Disable driver signature enforcement at boot

If you don't want the watermark or can't boot normally, use this one-time bypass:

1. Restart your PC.
2. As it boots, press F8 (or Shift+F8 on some systems) to enter Advanced Boot Options.
3. Select "Disable Driver Signature Enforcement."
4. Boot into Windows. This is a one-shot — it resets after reboot.

This is perfect for testing if a specific driver causes the issue without permanently changing your boot config.

Option 2: Find and replace the offending driver

The real fix is to swap out the bad driver. Here's how to track it down:

1. Press Win+R, type eventvwr.msc, and hit Enter.
2. Go to Windows Logs → System.
3. Look for BugCheck or Error events around the time of the crash. The description often includes the driver name (e.g., rt640x64.sys).
4. Once you have the driver file name, search for its source. Update it from the manufacturer's site or roll back to an earlier version.

For example, if it's nvlddmkm.sys, update your NVIDIA driver. If it's a third-party antivirus driver like mfehidk.sys, update or uninstall that security software.

Option 3: Run the System File Checker and DISM

Sometimes the assertion failure is because the driver file itself is corrupted, not just unsigned. Run these commands from an admin prompt:

sfc /scannow

dism /online /cleanup-image /restorehealth

Then reboot. This fixes system files that might be interfering with driver loading.

How to prevent this going forward

Once you've got your machine stable, take these steps so you never see 0xC0000420 again:

• Always download drivers from the hardware vendor's site — not from third-party updater tools or generic driver packs. Those often include unsigned or modified drivers that trigger assertion failures.
• Keep Windows updated. Microsoft patches driver verification mechanisms regularly. A newer version of Windows might handle assertions more gracefully.
• If you're a developer writing drivers, sign them properly. Use an EV certificate for production. For testing, use test signing mode and sign with your test certificate. Never release an unsigned driver.
• If you're using test signing mode long-term, be aware that some anti-cheat software (like in Valorant or League of Legends) will refuse to run. Turn test signing off before gaming.

That's it. The error is annoying, but it's also a sign — Windows is telling you a driver isn't playing by the rules. Listen to it, swap the driver, and you'll be back online fast.