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How to Fix c000021a Error on Windows 11: Step-by-Step Troubleshooting Guide

The c000021a error is a common Blue Screen of Death (BSOD) issue in Windows 11. It often appears suddenly and can prevent your computer from booting properly. This error usually points to a system process failure or corruption.

Fixing this error can seem intimidating, but with some basic troubleshooting steps, you can resolve it yourself. This guide will walk you through simple and clear solutions.

Each step is explained with the why and how, so even beginners can follow along without confusion. Let’s get started and get your Windows 11 back to normal.

Remember to be patient and try the steps in order, as they progress from easy fixes to more advanced methods.

Quick Note Before You Begin

  • Make sure you have backed up any important data if possible. Some steps may require system restore or recovery tools.
  • Have your Windows 11 installation media or recovery USB ready if you need to access advanced repair options.
  • Ensure your PC is plugged into power to avoid interruptions during troubleshooting.
  • Be ready to follow on-screen instructions carefully during repair or recovery processes.

Step 1: Restart Your Computer

Sometimes, a simple restart can clear temporary glitches causing the error. Press the power button to turn off your PC, wait about 30 seconds, then turn it on again.

If Windows boots normally after the restart, the issue may have been a one-time problem. If the error persists, move on to the next step.

Step 2: Boot into Safe Mode

Safe Mode starts Windows with only essential drivers and services. This helps identify if a third-party app or driver is causing the problem.

  1. Turn off your PC completely.
  2. Turn it on and as Windows starts loading, press the power button again to force shutdown. Repeat this 2-3 times until the “Preparing Automatic Repair” screen appears.
  3. Let the Automatic Repair process start, then select Advanced options.
  4. Go to Troubleshoot > Advanced options > Startup Settings.
  5. Click Restart. After restart, press 4 or F4 to enter Safe Mode.

Once in Safe Mode, check if the error appears again. If not, it’s likely caused by software or drivers.

Step 3: Run System File Checker (SFC) and DISM Scans

Corrupted system files can trigger the c000021a error. Windows provides built-in tools to scan and repair these files.

  1. While still in Safe Mode, press Windows + S, type cmd, right-click on Command Prompt, and select Run as administrator.
  2. In the command prompt window, type the following command and press Enter:
    sfc /scannow
  3. Wait for the scan to complete. It will automatically fix detected issues.
  4. Next, run the Deployment Image Servicing and Management tool by typing:
    DISM /Online /Cleanup-Image /RestoreHealth
  5. Again, wait for it to finish. This command repairs Windows system image corruption.
  6. After both scans finish, restart your computer normally and check if the error remains.

Step 4: Uninstall Recent Updates or Software

If the error began after installing new Windows updates or software, these could be the cause. Removing them can fix the problem.

  1. Boot into Safe Mode again (refer to Step 2).
  2. Press Windows + I to open Settings.
  3. Go to Windows Update > Update history.
  4. Scroll down and click Uninstall updates.
  5. Find the most recent updates installed before the error started and uninstall them one by one.
  6. If you installed new software recently, uninstall it via Apps > Installed apps.
  7. Restart your PC after uninstalling and test if the error is gone.

Step 5: Use System Restore

System Restore lets you revert your PC to a previous working state without affecting personal files. This can undo changes that caused the error.

  1. Access the Advanced Startup Options (as in Step 2).
  2. Go to Troubleshoot > Advanced options > System Restore.
  3. Select a restore point dated before the error appeared.
  4. Follow the on-screen instructions to complete the restore process.
  5. Your PC will restart and hopefully boot without the c000021a error.

Step 6: Repair Windows Using Installation Media

If previous steps don’t work, repairing Windows with the installation media can fix deeper system corruption.

  1. Create a Windows 11 bootable USB using Microsoft’s Media Creation Tool (if you don’t have one).
  2. Insert the USB and boot your PC from it (you may need to change boot order in BIOS).
  3. On the setup screen, click Repair your computer at the bottom left.
  4. Choose Troubleshoot > Advanced options > Startup Repair.
  5. Follow the prompts and allow Windows to attempt automatic repair.

This process fixes many system issues without deleting your files.

Alternative Method: Reset This PC

If repairing doesn’t help, you can reset Windows 11. This reinstalls the OS but lets you choose whether to keep your personal files.

  1. Access Advanced Startup Options.
  2. Select Troubleshoot > Reset this PC.
  3. Choose either Keep my files or Remove everything depending on your preference.
  4. Follow the instructions to complete the reset.

Note that Remove everything will delete all your files, so back up important data first.

Frequently Asked Questions (FAQs)

What exactly causes the c000021a error?

This error occurs when critical system processes fail or become corrupted. It can be triggered by faulty updates, corrupted system files, incompatible drivers, or malware.

Can I fix c000021a without losing data?

Yes. Steps like Safe Mode troubleshooting, SFC/DISM scans, uninstalling updates, or System Restore do not delete personal files. Only resetting or reinstalling Windows may affect data.

Is it safe to use third-party repair tools for this error?

While some tools can help, it’s best to use built-in Windows utilities first. Third-party tools can sometimes cause more problems or security risks.

How can I prevent this error in the future?

Keep Windows updated, avoid installing untrusted software, regularly scan for malware, and create system restore points before major changes.

What if my PC doesn’t boot at all?

Use Windows installation media to access recovery options or consider professional help if hardware issues are suspected.

When Nothing Works

If none of the above steps fix the c000021a error, it might signal severe system corruption or hardware failure. Here are your options:

  • Contact Microsoft Support for official assistance: https://support.microsoft.com
  • Consider taking your PC to a certified technician for hardware diagnostics.
  • Backup your data using a bootable Linux USB or external enclosure if possible, then perform a clean Windows reinstall.

Remember, persistent BSOD errors can sometimes indicate failing hardware like RAM or hard drives. Testing your hardware components may be necessary.

Conclusion

The c000021a error on Windows 11 can be frustrating but is usually fixable through step-by-step troubleshooting. Start by restarting and booting into Safe Mode, then scan and repair system files.

Uninstall recent updates or use System Restore to undo problematic changes. If needed, use Windows installation media for repairs or reset your PC.

Following these steps carefully should help you resolve the error without data loss. Always keep backups and maintain your system to reduce the chance of future errors.

With patience and the right approach, you can get your Windows 11 PC running smoothly again.

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