Microsoft Windows Error Code 0xC0000021: How to Fix It

Medium 30-60 minutes Medium Severity Verified June 2026
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Error Code
0xC0000021
Brand
Microsoft Windows
Product Type
operating_system
Severity
Medium
DIY Difficulty
Medium
Estimated Fix Time
30-60 minutes
Error code 0xC0000021 indicates that the memory was already committed — a condition where the system tries to allocate memory that is already in use. This usually appears during boot or when running memory-intensive applications, and can be caused by faulty hardware, corrupted system files, or driver issues.
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Tools You'll Need

How to Fix Error Code 0xC0000021

  1. Perform a Clean Boot

  2. Run Memory Diagnostics

    Back up important data before running diagnostics, as errors may indicate failing hardware.

  3. Check for Corrupted System Files

  4. Update or Roll Back Drivers

    Ensure you have a system restore point before rolling back drivers.

  5. Test RAM Sticks

    Unplug the computer and ground yourself to prevent static discharge.

Parts You May Need

RAM stick (if faulty)
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RAM stick (if faulty)
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RAM stick (if faulty)
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RAM stick (if faulty)
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When to Call a Professional

If memory diagnostics consistently show errors or replacing RAM doesn't fix the issue, a professional technician should check the motherboard or other hardware. Also, if you are not comfortable opening the computer case, seek assistance.

Frequently Asked Questions

What does error code 0xC0000021 mean?
It means that a program or the system tried to commit memory that was already committed, leading to a conflict. This often points to a memory allocation bug, faulty RAM, or driver issues.
Can I fix error 0xC0000021 without reinstalling Windows?
Yes, in most cases you can fix it with system file checker, clean boot, memory diagnostics, or driver updates. Reinstalling Windows is a last resort.
Is error 0xC0000021 caused by a virus?
It's not typically caused by a virus, but malware can cause memory corruption. It's a good idea to run a full antivirus scan to rule out infection.
How do I check if my RAM is failing?
Run the Windows Memory Diagnostic tool (mdsched.exe) or use a third-party tool like Memtest86. You can also physically test each RAM stick individually.
Will updating BIOS help fix error 0xC0000021?
In rare cases, a BIOS update can resolve memory compatibility issues. Check your motherboard manufacturer's website for updates, but proceed with caution as improper BIOS updates can cause problems.

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