How to Fix Windows Update Error 0x800f0991 [2026 Latest Guide]

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How to Fix Windows Update Error 0x800f0991 [2026 Latest Guide]

If you’re encountering the “Install error – 0x800f0991” message when trying to apply Windows 11 updates, you’re not alone. This error has been widely reported since the January 2026 security update (KB5074109), affecting numerous Windows 11 users. This article provides a thorough explanation of the causes behind error 0x800f0991 and offers step-by-step solutions that even beginners can follow. We’ve compiled proven fixes shared on Microsoft’s official Q&A forums and international tech communities, so if you’re struggling with this error, read on for help.

What Is This Error? Symptoms You’ll Experience

Windows Update error 0x800f0991 occurs when the Windows Component Based Servicing (CBS) engine fails to parse XML metadata during the installation of an update. Its official internal name is CBS_E_XML_PARSER_FAILURE, indicating that the Windows Update Agent cannot correctly read manifest files.

When this error occurs, the cumulative update download and installation process halts in the “Settings” > “Windows Update” screen, displaying the message “Install error – 0x800f0991.” The installation progress may freeze at a certain percentage, or after a reboot you might see the message “We couldn’t configure the Windows update. Reverting changes.”

On the Microsoft Q&A forum, this error has been reported in concentrated numbers for the January 2026 security update KB5074109, with Windows 11 version 24H2 and 25H2 users particularly affected. Similar errors have also been confirmed on Windows Server 2025. Since security updates remain unapplied, known vulnerabilities stay unpatched, making prompt resolution essential.

Why This Error Occurs

Cause 1: Corruption in the Windows Update Component Store

The most common cause is file corruption within the Windows Update component store (the WinSxS folder). The component store manages critical system files and update information. When the XML manifest files stored here become corrupted, the CBS engine can no longer correctly parse the update package contents, triggering the 0x800f0991 error.

This corruption can be caused by interrupted Windows Updates, improper shutdowns, disk issues, and various other factors. Windows 11 Forum widely recognizes component store corruption as the primary cause of this error.

Cause 2: Corrupted SoftwareDistribution Folder Cache

Update files downloaded by Windows Update are temporarily stored in the C:\Windows\SoftwareDistribution folder. When data in this cache folder becomes corrupted, downloaded update packages cannot be properly extracted, leading to installation errors.

According to Windows Report, the WIM files delivered via Windows Update were sometimes significantly smaller than those downloaded directly from the Microsoft Update Catalog, and incomplete downloads were confirmed as a cause of the error.

Cause 3: Damaged System Files

When core Windows OS system files are damaged for any reason, update installations may fail. In particular, when DLL files or registry information become inconsistent, the Servicing Stack cannot function properly, resulting in the CBS engine’s XML parser returning an error.

Cause 4: Missing Servicing Stack Update (SSU)

The Servicing Stack Update (SSU) is a critical component that updates the Windows Update mechanism itself. When the SSU is not up to date, prerequisites for installing new cumulative updates are not met, which can trigger the 0x800f0991 error. The Microsoft Community Hub has also identified missing SSUs as one of the causes.

Cause 5: Third-Party Software Interference

Antivirus or firewall software may block Windows Update communications or file operations, preventing update downloads or installations from completing normally. This error is particularly likely when real-time protection features restrict writes to Windows system folders.

Fix 1: Reset Windows Update Components (Recommended)

This method is the most recommended solution on Microsoft Q&A. By stopping Windows Update-related services and renaming cache folders, you force a completely fresh download of update metadata.

Step 1: Open Command Prompt as Administrator

Type “cmd” in the Start menu search bar, right-click on “Command Prompt,” and select “Run as administrator.” Click “Yes” when the User Account Control (UAC) prompt appears.

Step 2: Stop Windows Update-Related Services

Enter the following four commands one at a time, pressing Enter after each. This stops all Windows Update-related services.

net stop wuauserv
net stop cryptSvc
net stop bits
net stop msiserver

Each command will display “The service was stopped successfully.” If a service is already stopped, you’ll see “The service is not started” — this is not a problem.

Step 3: Rename Cache Folders

Execute the following two commands to rename the update cache folders. Windows will create new folders and rebuild the cache from scratch on the next update attempt.

ren C:\Windows\SoftwareDistribution SoftwareDistribution.old
ren C:\Windows\System32\catroot2 catroot2.old

Step 4: Restart the Services

Restart all previously stopped services with these commands:

net start wuauserv
net start cryptSvc
net start bits
net start msiserver

Step 5: Retry Windows Update

Open “Settings” > “Windows Update” and click “Check for updates” to attempt the update installation again. In many cases, this resolves the error.

Important Notes

  • Commands must be run in an elevated Command Prompt. A regular Command Prompt will return “Access is denied.”
  • The renamed folders (SoftwareDistribution.old, catroot2.old) can be safely deleted after the issue is resolved.
  • This operation clears all previously downloaded update caches, requiring updates to be re-downloaded.

Fix 2: System File Repair with DISM and SFC

If Fix 1 doesn’t resolve the error, use the DISM (Deployment Image Servicing and Management) tool and SFC (System File Checker) command to verify and repair system file integrity. Sysnative Forums has numerous reports of this method successfully resolving the error.

Step 1: Run DISM Commands for Image Repair

Open an elevated Command Prompt and execute the following commands in order. Each command may take several minutes to tens of minutes to complete — do not interrupt the process.

Dism /Online /Cleanup-Image /StartComponentCleanup
Dism /Online /Cleanup-Image /CheckHealth
Dism /Online /Cleanup-Image /ScanHealth
Dism /Online /Cleanup-Image /RestoreHealth

StartComponentCleanup cleans up unnecessary files in the component store, CheckHealth and ScanHealth verify image integrity, and RestoreHealth automatically downloads and repairs corrupted files through Windows Update when damage is detected.

Step 2: Run SFC Scan

After the DISM commands complete, run the following command:

sfc /scannow

This command scans all Windows system files and automatically replaces corrupted files with correct copies. If you see “Windows Resource Protection found corrupt files and successfully repaired them,” the repair was successful.

Step 3: Restart and Retry the Update

After all commands have completed, restart your PC. Then go to “Settings” > “Windows Update” and try installing the update again.

Fix 3: In-Place Upgrade (Repair Install)

For the most stubborn cases where the above methods don’t work, an in-place upgrade (Repair Install) is the most reliable solution. This method is officially recommended on Microsoft Learn and completely reinstalls Windows system files while preserving your personal files and installed applications.

Step 1: Download the Windows 11 ISO File

Download the Windows 11 ISO file from the official Microsoft website. Select the version that matches your PC’s architecture (x64).

Step 2: Mount the ISO File

Double-click the downloaded ISO file to mount it as a virtual drive. Open the newly displayed drive in File Explorer.

Step 3: Run setup.exe

Double-click setup.exe inside the mounted drive. Click “Yes” when the User Account Control prompt appears.

Step 4: Select “Keep personal files and apps”

When the setup wizard appears, accept the license terms, click “Change what to keep,” and select “Keep personal files and apps.” This setting preserves your personal data and installed applications while reinstalling only the core Windows system files.

Step 5: Start the Installation

Click the “Install” button to begin the repair installation. Your PC will restart several times during the process — this is normal. The entire process may take 30 minutes to an hour.

Important Notes

  • Strongly recommended to back up important data before performing an in-place upgrade.
  • Temporarily uninstall or disable third-party antivirus software.
  • At least 20 GB of free space is required on the system drive.
  • After the repair install, a Windows.old folder will be created. If everything works correctly, you can delete it via “Settings” > “System” > “Storage” > “Temporary files.”

How to Prevent This Error

To prevent error 0x800f0991 from recurring, we recommend implementing the following preventive measures regularly.

First, keep Windows Update automatic updates enabled and apply updates promptly when they become available. Leaving updates pending for long periods increases the likelihood of inconsistencies between old and new components.

Second, periodically run Disk Cleanup to delete old Windows Update cache files. Go to “Settings” > “System” > “Storage” > “Temporary files” and select “Windows Update Cleanup” to safely remove unnecessary caches.

Additionally, make it a habit to run the sfc /scannow command in an elevated Command Prompt about once a month to check system file health. Catching problems while they’re small can prevent major issues.

Furthermore, never turn off your PC or force a shutdown during update installation. Interrupted installations are the leading cause of component store corruption. If you’re using a laptop, make sure the AC adapter is connected during update installations.

Summary

Windows Update error 0x800f0991 (CBS_E_XML_PARSER_FAILURE) has been widely reported among Windows 11 users since the distribution of the January 2026 security update KB5074109. The primary causes are corruption in Windows Update’s component store and cache, and in most cases, the issue can be resolved through a step-by-step approach.

The most effective order of remediation is to first try Fix 1: Resetting Windows Update Components, then if that doesn’t work, proceed to Fix 2: System File Repair with DISM/SFC commands, and for the most persistent cases, Fix 3: In-Place Upgrade (Repair Install) provides a definitive solution.

Note that Microsoft has released an emergency patch KB5078127 to address multiple issues introduced in the January 2026 KB5074109 update. This patch also includes fixes for apps like Outlook and OneDrive crashing, so if you haven’t applied it yet, download it from the Microsoft Update Catalog.

If you still can’t resolve the issue, post a question on the Microsoft Q&A forum or contact Microsoft support directly.

References

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