Fix STATUS_REPLY_MESSAGE_MISMATCH (0xC000021F) on Windows

What 0xC000021F Actually Means

The error code 0xC000021F translates to STATUS_REPLY_MESSAGE_MISMATCH. It's a kernel-level failure in the Local Procedure Call (LPC) subsystem. In plain English: one part of Windows sent a message to another part, and the reply didn't match what was expected. That mismatch usually happens because something in the middle — a driver, a security tool, or corrupted system files — altered or intercepted the message.

You'll typically see this error as a blue screen (BSOD) with the message STATUS_REPLY_MESSAGE_MISMATCH. It can also appear as a boot failure or a random restart. I've seen it most often on machines running third-party antivirus alongside Windows Defender, or after a driver update that didn't quite fit.

Cause 1: Third-Party Antivirus or Security Software

This is the number one culprit. Antivirus programs hook into system calls to scan them. If the hooking is buggy or conflicts with a Windows update, the LPC reply gets mangled. The mismatch triggers the bugcheck.

Fix: Boot into Safe Mode, then uninstall the antivirus completely. Don't just disable it — uninstall it. Use the vendor's removal tool if they have one (Norton, McAfee, and Avast all provide cleanup utilities). After uninstalling, restart normally. If the error stops, you've found the cause. You can reinstall a different antivirus or stick with Windows Defender, which is plenty for most people.

To get into Safe Mode: hold Shift while clicking Restart from the Start menu. Then go to Troubleshoot > Advanced options > Startup Settings > Restart. Press 4 or F4 for Safe Mode.

Cause 2: Corrupted System Files or Windows Update Components

If antivirus isn't the issue, the next likely cause is corrupted system files. A bad update or a failed installation can leave core Windows files in a broken state. The LPC subsystem relies on these files, so a single corruption can cause the mismatch.

Fix: Run System File Checker and DISM. Open Command Prompt as Administrator. Type:

sfc /scannow

Let it finish. If it finds and fixes errors, restart and see if the error returns. If SFC can't fix it, run DISM:

DISM /Online /Cleanup-Image /RestoreHealth

After DISM completes, run sfc /scannow again. This combo repairs the component store and then replaces bad files. I've fixed dozens of 0xC000021F cases this way after a botched cumulative update.

Cause 3: Faulty or Outdated Drivers

Drivers are the third most common cause. A driver that doesn't handle LPC messages correctly can cause the mismatch. This is especially common with network drivers, storage drivers, and any driver from a small vendor that hasn't been updated for your Windows build.

Fix: Update your drivers, but do it selectively. Don't use a driver updater tool — they cause more problems than they solve. Go to your PC manufacturer's support site and download the latest chipset, network, and storage drivers for your exact model. If you recently updated a driver and then got the error, roll it back: open Device Manager, find the device, right-click, Properties, Driver tab, and click Roll Back Driver.

If you can't boot at all, use Safe Mode to roll back or uninstall the suspect driver.

Cause 4: RAM or Hardware Issues

Less common, but possible. Bad RAM can corrupt the memory where LPC messages are stored. If you've tried the software fixes and still get 0xC000021F, test your memory with Windows Memory Diagnostic. Type mdsched.exe in the Run box and choose to restart now. The test runs at boot.

If it reports errors, replace the faulty stick. Also check your hard drive or SSD with chkdsk /f /r from an elevated Command Prompt.

Quick Reference: Fixes by Cause

Start with the antivirus check. It's the fastest and most likely fix. If that doesn't do it, move to system file repair. Drivers and hardware come last, but don't skip them if the error persists. And if you're ever unsure, create a restore point before making changes.