How to Fix C000021A Error on Windows 7 / Windows 8.1 (Fatal System Error)

Some Windows 7 and Windows 8.1 users are encountering the Stop Error code C000021A dooring the bootup sequence. In most cases, affected users are reporting that the problem started occurring after they’ve updated their BIOS version.

Fatal System Error C000021A

After investigating the issue, it turns out that there are several different causes that might trigger the c000021a error on a Windows computer. Here’s a list of potential culprits:

  • Underlying storage issue – As it turns out, this issue can also be caused by a logical error or another type of storage-related problem. In this case, your best chance at fixing the error code is to deploy a CHKDSK scan in order to fix issues related to your HDD or SSD.
  • System File corruption – Under certain circumstances, you can face this issue due to some kind of system file corruption that ends up affecting the stability of your OS. If this scenario is applicable, you should be able to fix the problem in most cases by running a couple of utilities capable of resolving Windows File corruption (DISM and SFC). If that’s not enough, you’ll need to go for a more transformative procedure like a clean install or repair install.
  • Corrupted BCD data – According to some affected users, this particular error code can also be related to corruption among your Boot Configuration Data. In this case, the only way to fix the problem is to rebuild the BCD data from an elevated CMD prompt using a series of commands.
  • Recent OS change – If you only started to see this problem recently, chances are you are facing this problem due to a recent change that a driver, update, or application has made to your system. To fix it, you will need to use a system restore point to bring your computer back to a healthy state.

Method 1: Running a CHKDSK scan

Before you try any of the other fixes below, you should start by investigating a HDD / SSD storage-related problem. In this case, a CHKDSK  scan using the Check Disk utility should allow you to fix the issue in the vast majority of cases.

Doing this will ensure that you eliminate every instance of a bad SSD / HDD sector that will be eliminated which might fix the c000021a error code.

Keep in mind that the CHKDSK is present on every recent Windows version including Windows 7 and Windows 8.1, so you will be able to initiate a Check Disk scan regardless of your windows version. This has the potential to fix the majority of corruption instances of logical errors that might contribute to the Stop error code.

To initiate the most effective CHKDSK scan possible, you need to do it from an elevated Command prompt. In case you haven’t done this before following these instructions on running a CHKDSK scan from an elevated CMD.

Running a chkdsk command

If you already did this and you’re still seeing the same error code, move down to the next potential fix below.

Method 2: Running SFC and DISM scans

If the Check Disk utility didn’t fix the issue in your case, the next thing you should do is utilize a couple of utilities (SFC and DISM) that are capable of fixing system file corruption that might cause general OS instability.

System File Checker and Deployment Image Servicing and Management are 2 built-in tools that are equipped to fix the most common corruption instances that might trigger the c000021a error code.

In case you haven’t done this yet, start by going for a System File Checker scan since you can do it without requiring an active internet connection.

Running SFC Utility

Note: SFC works by using a locally stored cache to replace corrupted system file items with healthy equivalents. It’s very important to avoid interrupting this SFC scan once you initiate it – Causing an interruption has the potential of creating additional logical errors.

IMPORTANT: If you’re using a traditional HDD instead of a more modern SSD, you can expect this operation to take more than 1 hour. This utility has a tendency to freeze temporarily – If this happens, don’t close the window and wait for the real-time tracking to return.

When the SFC scan is finally complete, reboot your computer and see if the c000021a error code is finally fixed.

In case it isn’t, move forward by initiating a DISM scan.

Run DISM Command

Note: DISM uses a component of Windows Update to replace corrupted Windows file instances with healthy copies downloaded over the internet. Because it relies on an internet connection, you need to ensure that your connection is stable before starting this operation.

Once the operation is complete, reboot your computer and use your computer normally to see if you still end up seeing the c000021a error code.

In case the problem persists, move down to Method 3.

Method 3: Rebuilding the BCD Data

As it turns out, this problem can also be caused by corrupted BCD data that is interfering with the startup operation. If this scenario is applicable, you should be able to fix the issue by using an elevated Command Prompt to rebuild the Boot Configuration Data and fix the startup sequence.

This method was confirmed to work by a lot of affected users (especially if the issue is occurring at every conventional startup attempt)

If you don’t know how to do this, follow the instructions below to initiate a BCD repair procedure from the Troubleshoot menu of your Windows version:

Note: The instructions below will work even if you are not able to boot your computer conventional anymore.

Here’s a quick step-by-step guide on rebuilding the BCD data using the Troubleshooting menu:

  1. First things first, insert a compatible installation Windows installation media and press any key to make your computer boot from it.
    Press any key to boot from installation media
    Press any key to boot from installation media
  2. Once your computer successfully boots from the Windows installation media disc, click on Repair your computer.

    Click Repair your computer on Windows Screen
  3. Once you’re inside the Recovery menu, click on Troubleshoot, then click on Command Prompt from the list of available options.
    Click Command Prompt

    Note: If you open the Command Prompt from the recovery menu, it will automatically open with admin rights.

  4. Inside the Command prompt window, type the following commands and press Enter after each one to start the process of rebuilding the Boot Configuration Data:
    Bcdedit /export C:\BCD_Backup
    ren c:\boot\bcd bcd.old
    Bootrec.exe /rebuildbcd
    BootRec.exe /fixmbr
    BootRec.exe /fixboot
    sfc /scannow /offbootdir=c:\ /offwindir=c:\windows
    chkdsk c: /r
  5. Wait until the operation is complete, then restart your computer and see if your computer is able to boot conventionally once the next startup is complete.

If the same problem is still occurring, move down to the next method below.

Method 4: Using a System Restore point

If none of the methods above have fixed the issue for you and the issue has started occurring after you undertake a certain action, you might be able to fix the issue by using the System Restore utility.

System Restore is a utility that is configured by default to create regular snapshots of your computer state. If something goes wrong, you can use them to return your computer back to a state in which your PC was functioning properly.

Note: Unless you modified the default behavior of System Restore (or a resource management tool did it for you), you should have plenty of snapshots to choose from – By default, this utility is configured to create new snapshots at important system events such as the installation of an update, a new driver version, etc.

If you want to utilize an older system to restore a snapshot to get around the c000021a error, keep in mind that once you do so, every installed application, driver, game, or another kind of system change will be reverted.

If you understand how this utility works and what it can do for you, follow these instructions to initiate a System Restore procedure.

Using System Restore

If the same problem is still occurring, move down to the next method below.

Method 5: Clean installing / Repair installing

If you followed every potential fix above and none of them allowed you to fix the c000021a error code, chances are you’re dealing with some kind of system file corruption that cannot be resolved conventionally with built-in utilities like SFC or DISM.

If this scenario is applicable, you’ll be able to fix the problem by resetting every Windows component with a clean install or repair install procedure.

If you’re looking for the easiest fix, you should go for the clean install, but keep in mind that this procedure doesn’t come without compromises. If you go this route, you can expect to lose any personal files (media, apps, games) that you’re currently on your OS drive that isn’t backed up. However, the main advantage is that you don’t need compatible installation media to initiate this operation.

Clean Installing Windows

However, if you’re after the focused approach, you will need compatible installation media in order to initiate a repair install (in-place repair procedure). If you compare this operation with a clean install, it’s considerably more tedious, but the main advantage is that you get to keep all your personal files present on the OS drive (personal media, applications, games, and even some user preferences.

Kevin Arrows
Kevin is a dynamic and self-motivated information technology professional, with a Thorough knowledge of all facets pertaining to network infrastructure design, implementation and administration. Superior record of delivering simultaneous large-scale mission critical projects on time and under budget.

Expert Tip

How to Fix C000021A Error on Windows 7 / Windows 8.1 (Fatal System Error)

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