Windows update ( KB5066835 ) ( 26100.6899 ) Error ( 0x800f0991 )

Question

Hello all,

I try to install the update: (KB5066835) (26100.6899) and Windows 11 version 25H2, but it wont install. I keep getting the error: ( 0x800f0991 ).

I did some research on the forum but nothing fixed.

Fix 1 - Failed:

sfc /scannow
Dism /Online /Cleanup-image /ScanHealth
Dism /Online /Cleanup-image /CheckHealth
Dism /Online /Cleanup-image /RestoreHealth
Dism /Online /Cleanup-image /StartComponentCleanup

Fix 2 - Failed:

Manually downloaded the 25H2

Please help me out.

Answer 1

Try upgrading to version 25H2 by doing an In-place Upgrade using an ISO file.

But don't worry. It's a simple procedure, and you have the option to keep files, settings, and apps. Therefore, all your data is kept.

---

Download the ISO file to perform the procedure:

1. Access the link below;

https://www.microsoft.com/en-us/software-download/windows11

2. Under "Download Windows 11 Disk Image (ISO)", click "Select download" and select "Windows 11 (multi-edition ISO for x64 devices)";
3. Click "Download";
4. Choose the language and click on "Confirm";
5. Then, click "64-bit Download".
6. Wait for the download to finish and proceed to the procedure below.

---

After downloading the ISO, do the following:

1. Double-click the ISO to run;

(After running the ISO, you may need to double-click the "Setup" file to start the procedure.)

2. On the "Install Windows 11" screen, uncheck "I want to help make the installation of Windows better" and select "Change how Windows Setup download updates";
3. On the next screen, select "Not right now" and click "Next";
4. Accept the terms of use;
5. On the "Ready to install" screen, select "Choose what to keep" and select what you want to keep.

(It is in this step that you must check "Keep personal files and apps" so that all your data is kept.)

6. Click on "Install" and wait for the procedure to finish. Your computer may restart several times.

---

At the end of the procedure, your Windows 11 will be in the latest version (25H2), and some updates may already be installed. But check Windows Update to be sure.

---

If you get any errors, please let me know.

---

Feel free to ask back any questions.

Answer 2

The error code 0x800f0991 typically indicates that there are missing files or components necessary for the update installation. Since you've already tried several methods without success, here are a few additional steps you can take:

1. Ensure Windows Update Service is Running: Make sure that the Windows Update service is running. You can do this by typing services.msc in the Run dialog (Win + R), finding "Windows Update" in the list, and ensuring it is set to "Running".
2. Check for Corrupted Windows Update Components: You can reset the Windows Update components by running the following commands in an elevated Command Prompt:

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

3. Use DISM to Repair Windows Update: Since you have already used DISM commands, ensure you are connected to the internet when running:

      DISM.exe /Online /Cleanup-image /RestoreHealth
   

   This command can fetch missing files from Windows Update.
4. Manual Installation of the Update: If the update continues to fail, you can try manually downloading and installing the update from the Microsoft Update Catalog. Make sure to select the correct version for your system.
5. Check CBS.log: If issues persist, check the CBS.log file located at %windir%\Logs\CBS\CBS.log for any specific errors that might give more insight into what is causing the update to fail.

If these steps do not resolve the issue, you may want to consider performing a repair install of Windows 11, which can fix system issues without affecting your files.

---

References:

• Repair a Windows Image
• Using DISM to repair Windows Update corruptions

Answer 3

Hi. I'm David, and I'm happy to help you.

---

Please clarify the following:

When you say "Manually downloaded the 25H2," did you use the Windows 11 ISO and perform an in-place upgrade?

Do you have a third-party antivirus installed? If yes, which one?

Also. What is the current "OS Build" of your Windows 11?

• Press "Windows + R", type winver, and click OK.

If you prefer, share a screenshot. Use the shortcut "Windows + Shift + S" to use the screenshot tool.

Be sure to censor your email address if it appears. The Microsoft Q&A is public.

---

I await your reply.