Windows Update failures with error code 0x800f0991 often occur due to missing system files, corrupted update components, or conflicts with installed applications. This error blocks cumulative and security updates, leaving your system without the latest features and security patches. The following methods address the root causes and restore normal update functionality.

Perform an In-Place Upgrade

Running an in-place upgrade repairs system files and reinstalls Windows while keeping your apps, settings, and personal files intact. This approach directly addresses underlying corruption or missing dependencies that typical troubleshooting steps may not resolve.

Step 1: Download the latest Windows 11 ISO file from the official Microsoft website or use a trusted tool like uupdump.net to create a new ISO containing the most recent cumulative update. This ensures that the installer includes the files required for your system version.

Step 2: Disconnect all external devices except for your keyboard, mouse, and network cable. Uninstall or temporarily disable any third-party antivirus software to avoid interference during installation.

Step 3: Right-click the ISO file and select Mount. Open the mounted drive and run setup.exe. Follow the prompts, select the option to keep your files and apps, and proceed through the installation. The process may require several restarts and can take some time.

Step 4: Once the upgrade completes, check for Windows Updates again. The error should no longer appear, and updates should install successfully.

Reset Windows Update Components

Corrupted or stuck Windows Update components can prevent updates from installing. Resetting these components forces Windows to rebuild update-related folders and services.

Step 1: Open Command Prompt as an administrator by searching for cmd in the Start menu, right-clicking it and selecting Run as administrator.

Step 2: Stop the update-related services by entering the following commands one at a time:

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

Step 3: Rename the update storage folders so Windows can recreate them:

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

Step 4: Restart the services you stopped earlier:

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

Step 5: Close the Command Prompt and restart your computer. Try installing updates again to see if the issue is resolved.

Run System File Checker and DISM Tools

Corruption in system files or the Windows component store can trigger update errors. System File Checker (SFC) and Deployment Imaging Service and Management Tool (DISM) are built-in utilities that scan and repair these problems.

Step 1: Open Command Prompt as an administrator.

Step 2: Run the System File Checker by entering:

sfc /scannow

This process scans for and repairs corrupted or missing system files. Wait for it to complete before proceeding.

Step 3: Use DISM to repair the Windows image by running each command in order:

DISM /Online /Cleanup-Image /CheckHealth
DISM /Online /Cleanup-Image /ScanHealth
DISM /Online /Cleanup-Image /RestoreHealth

These commands check for corruption, scan the system image, and attempt repairs. After completion, restart your computer and try updating again.

Install Updates Manually

When automatic updates fail, manually downloading and installing the update package can sometimes bypass the issue—especially if the update process is blocked by incomplete downloads or catalog mismatches.

Step 1: Go to the Microsoft Update Catalog and search for the specific update (for example, KB5036893 or KB5053598).

Step 2: Download the update that matches your Windows version and system architecture (x64 or ARM64).

Step 3: Double-click the downloaded file to run the installer. Follow the prompts to complete the installation. Restart your computer if required, and check if the update now appears as installed.

Additional Steps and Cautions

For persistent issues, check that critical services like Windows Update, Windows Installer, and Background Intelligent Transfer Service are running. You can verify and restart these from the services.msc management console. If you encounter permissions errors with specific folders (such as C:\Program Files\WindowsApps), ensure you have a complete backup before making any changes, as deleting or modifying these folders can affect app functionality.

Some users have reported that disk cleanup utilities may not remove all update remnants. If disk cleanup fails to clear update files, focus on the above repair methods instead of repeated cleanup attempts.

If all else fails and you continue to receive error 0x800f0991, a fresh installation of Windows 11 using the latest ISO may be necessary. Choose the installation option that retains your files and apps to minimize disruption.

Resolving error 0x800f0991 on Windows 11 restores update functionality and keeps your system secure. Regularly running system maintenance tools and keeping backups can prevent similar issues in the future.