KB5079473 on build 26200.8037 fails to install - error code 0x800f0991

Question

KB5079473 on build 26200.8037 fails to install - error code 0x800f0991

Rob 35

It downloads then attempts to install the abovementioned update. The installation stops and returns the error code given above. I have tried about ten times.

I have tried the troubleshooter (which says there are no issues), run a few DISM commands and an SFC. I am not running a third party virus checker. I have tried to download the update and run it manually but this also fails in the same way.

This is a newly built (last week) PC from a reputable company and I am running Win 11 Pro.

I have a CBS log if needed.

Is this a Microsoft issue that will be fixed? What else do I need to do to get this update?

sekah glass pro 0 Reputation points

2026-03-23T13:34:05.1433333+00:00

how to ubdate an Download the Windows

Answer accepted by question author

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sekah glass pro 0 Reputation points

2026-03-23T13:34:05.1433333+00:00

how to ubdate an Download the Windows

Answer 1

For KB5079473 (build 26200.8037) failing with 0x800f0991 on a new Windows 11 Pro PC, this behavior can be a transient Microsoft-side issue with the update itself. In the referenced case, after multiple failures and trying DISM, SFC, the troubleshooter, and manual installation, the update eventually installed successfully without any additional action once Microsoft corrected the underlying problem.

Given that DISM, SFC, and the Windows Update troubleshooter have already been run and the system is otherwise healthy, the next step is to wait and retry periodically. Allow Windows Update to attempt installation again over the next day or two; if it is the same issue, it should self-correct and install successfully once the backend or package issue is resolved.

If the update continues to fail over time, monitor Windows Update for any newer cumulative updates that supersede KB5079473 and install those instead, as they typically include the same fixes and may bypass the specific failure affecting this build.

References:

Latest Security Update KB5079473 (26200.8037) Fails to Install - Microsoft Q&A

Answer 2

Rob 35

So now there appears to be an OOB that's been pushed to me as available to download and install.

This also fails:

User's image

My OS build is Win 11 Pro 25H2 26200.7840

I'm not sure what's going on at Microsoft with these latest KB updates.

Norah-L 5,350 Reputation points Microsoft External Staff Moderator

2026-03-23T02:16:28.2533333+00:00

Hi Rob!

Thank you for posting back with an update.

Could you try downloading and installing the update manually from the Microsoft Update Catalog to see if it resolves the issue?
Rob 35 Reputation points

2026-03-23T10:08:04.9533333+00:00
Hi Norah, thanks for getting back to me.

I have visited the MS catalogue to download the KB. When I clicked on the download box, I was given two files:

I have downloaded both.

Three questions:

Do I install KB5043080 first, then KB5085516 second?

To install, I just double click the MSU file - is that correct?

Are there any operations I should perform before installation or does the install do everything automatically?

Many thanks!
Norah-L 5,350 Reputation points Microsoft External Staff Moderator

2026-03-24T07:00:05.6233333+00:00
Good day Rob!

1. Do I install KB5043080 first, then KB5085516 second?

Yes. Since they appear in the same package, please download both and install them in this order: KB5043080 first, then KB5085516.

2. To install, I just double click the MSU file - is that correct?

Yes, that’s correct.

3. Are there any operations I should perform before installation or does the install do everything automatically?

For most users, no special preparation is required. The installer handles everything automatically. If you run into any issues, try the built‑in Windows Update troubleshooter, then reset Windows Update components by running the following commands in an elevated Command Prompt:

net stop wuauserv net stop cryptSvc net stop bits ren C:\Windows\SoftwareDistribution SoftwareDistribution.old ren C:\Windows\System32\catroot2 catroot2.old net start wuauserv net start cryptSvc net start bits netsh winsock reset

Feel free to post back if you need further support!

Yours sincerely.
Rob 35 Reputation points

2026-03-24T09:39:38.7333333+00:00

Hello. I'm sorry, but none of this is working.

When trying to install KB504308 manually, this error occurred:

I then went ahead to manually install KB408516 manually anyway it just said the update was not installed.

During the reset components stage, I then encountered this error which may appear to be the issue:

I don't know what else to try.
Norah-L 5,350 Reputation points Microsoft External Staff Moderator

2026-03-25T06:42:20.7133333+00:00
Note: For privacy and security reasons, I have blurred some personal information. I sent the original screenshot via private message.

Nice day Rob!

Please try the following to clear any pending operations left behind by a previous failed update, so Windows can start with a clean state.

Open Command Prompt as Administrator and run:
DISM /Online /Cleanup-Image /RevertPendingActions

Restart the device once the command completes. Then return to Windows Update or the Catalog website and try installing KB5085516 again.

If the issue still persists, an in‑place upgrade should be considered at that point. Even though system scans have already been run, the error code 0x800f0991 suggests a possible component store related issue that an in‑place upgrade may help resolve.

To perform an in‑place upgrade:

Download the Windows 11 ISO using Microsoft’s Create Installation Media guide.

Double‑click the ISO file to mount it.

Run setup.exe from the mounted drive.

When prompted, select Keep personal files and apps.

Let the upgrade complete. This takes 20 to 45 minutes depending on your hardware.
Rob 35 Reputation points

2026-03-25T11:00:21.3633333+00:00

Morning.

I tried the /RevertPendingActions DISM comment, rebooted, then tried to install KB5085516 again from the MSU file. This failed again.

However, Windows update is now showing this KB plus KB5079473 as being pending and both failing with the error code previously supplied.

As you advised, my next attempt will be to install from the ISO but now I don't know which one to download/use.

Should it be KB5085516 or KB5079473? Or both?

It seems to have got in quite the mess!

Thanks.
Norah-L 5,350 Reputation points Microsoft External Staff Moderator

2026-03-26T05:05:00.6933333+00:00

Hi Rob!

Sorry for the earlier confusion. In this context, installing from the ISO means performing an in‑place upgrade to reinstall Windows and repair any corrupted system files that may be causing the issue. It is not intended for installing a specific Windows update.

For a step‑by‑step guide on how to do an in‑place upgrade, please refer to the linked article: [Article] How to: Perform a Repair Upgrade Using the Windows 11 or Windows 10 ISO file

You can download the official Windows 11 or Windows 10 ISO directly from Microsoft here: https://www.microsoft.com/en-us/software-download

Best regards.
Rob 35 Reputation points

2026-03-26T10:39:33.3066667+00:00

Ah, I see.

Looks simple enough :)

Before I begin, can I just clarify a couple of things? I have downloaded Win11_25H2_EnglishInternational_x64_v2.iso - I am running Win 11 Pro so will this install retain that version?

Secondly, the "English International" piece refers to UK English - is that correct? There only appears to be two English versions available: US and international.

Sorry for all the back and forth on this!

Thanks for your oatience!
Norah-L 5,350 Reputation points Microsoft External Staff Moderator

2026-03-29T08:07:14.8966667+00:00
Hello Rob!

I have downloaded Win11_25H2_EnglishInternational_x64_v2.iso - I am running Win 11 Pro so will this install retain that version?

Yes, an in‑place upgrade will retain your Windows 11 Pro edition as the ISO does not decide the edition. Its role is to provide the updated system files.

When you launch Setup from within Windows, Setup checks your current activation and automatically applies the same licensed edition (Pro) while upgrading to the newer build.

Secondly, the "English International" piece refers to UK English - is that correct? There only appears to be two English versions available: US and international.

The English International ISO includes localized English variants (such as UK, Australia, and other non‑US English regions...). Which one to use depends on your device setup region:

If your existing Windows is English (United States) > use the US (English) ISO

If your existing Windows is English (International / UK) > use the English International ISO

Hope this can clarify your concern!

Yours sincerely.
Rob 35 Reputation points

2026-03-30T13:08:53.5466667+00:00

Hi!

Just to let you know that I did the in-place repair install from the ISO and it all seems to be working now - KB5085516 has downloaded and installed successfully.

Many thanks for your help!
Norah-L 5,350 Reputation points Microsoft External Staff Moderator

2026-03-30T23:29:40.56+00:00

Thank you for the update, Rob!

I’m glad to hear everything is going well now.

Wishing you a great day!

Answer 3

Rob 35

Hi Norah-L

Unfortunately, your answer hasn't provided a fix.

I did not carry out the suggestion because:

The PC is completely new, having been built only two weeks ago by Scan.co.uk
I have run DISM and CHKDSK and results from both reported zero errors and zero corruption

Performing an in-place repair seems irrelevant in this case.

In addition, the troubleshooter for Windows updates reports no issues detected.

For convenience, I am re-attaching my CBSlog

Manu thanks.

Norah-L 5,350 Reputation points Microsoft External Staff Moderator

2026-03-18T05:44:03.9633333+00:00

Have a nice day, Rob!

I’m not seeing the CBS.log attached here. If possible, could you copy and paste the relevant details via private message?

Thank you very much for your patience.
Rob 35 Reputation points

2026-03-18T09:45:53.7066667+00:00

It seems the CBS log is too long to fit in this text box. I'll try to attach it again.
Rob 35 Reputation points

2026-03-18T10:21:13.2233333+00:00

I am attaching a pdf version of the file. I hope you can now see the log. If not, is there an email address I can send it to get around the restrictions on these forms?

Many thanks.
Rob 35 Reputation points

2026-03-20T10:46:57.4433333+00:00

Good Morning,

I haven't heard anything from you on this. After various internet searches, it seems the problem of the KB not installing is quite widely reported.

However, MS haven't acknowledged the issue as yet.

Will there be a patch release to fix this?

Also, were you able to see me CBSlog now?

If not, I don't know how else to get it to you.

Thanks.
Norah-L 5,350 Reputation points Microsoft External Staff Moderator

2026-03-22T05:15:48.96+00:00

Hi Rob!

I'm so sorry for the late response, I was away for a few days.

Based on community reports, similar behavior has been observed by other users as well. At this point, unfortunately there hasn’t been an official update or guidance on this specific issue.

In situations like this, the best approach is to keep an eye on official Windows Update release notes. If it is a widespread issue, it is typically addressed in a future cumulative update, most often released on Patch Tuesday (the second Tuesday of each month). The next update may either fix the problem directly or supersede the problematic KB.

About the CBS.log: I’m still not able to view it from here. If you’ve already tried sharing it and it didn’t come through, that’s okay. Since similar reports are being seen from other users as well, there may not be much more we can do log‑wise now.

For now, I’d recommend keeping Windows fully up to date, watching for the next cumulative update and checking official Microsoft communications for any acknowledgment or guidance.

If any new information becomes available regarding this issue, I’ll be sure to share it here.

Thank you so much for your patience!

Your sincerely.

Answer 4

Norah-L 5,350 Microsoft External Staff Moderator

Welcome Rob!

Thank you for reaching out.

From what you described, the KB5079473 update on Windows 11 build 26200.8037 does not complete the installation process and rolls back with error code 0x800f0991. You’ve attempted to install the update multiple times, but the behavior remains the same each time.

The error code 0x800f0991 during a Windows Update installation typically indicates that there are missing files or components needed for the update to proceed.

One effective approach is to perform an in‑place upgrade (repair upgrade), which reinstalls Windows without removing your files or apps. You can refer to this similar case with the same error code, where the issue was resolved using an in‑place upgrade: Latest Security Update KB5079473 (26200.8037) Fails to Install

To perform an in‑place upgrade, please see this guide: [How to perform a repair upgrade using the Windows 11 or Windows 10 ISO file]

You can download the official Windows 11 ISO directly from Microsoft here: https://www.microsoft.com/en-us/software-download/windows11

Hope this helps. Please let me know how it goes!

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Mark 0 Reputation points

2026-03-16T23:59:25.51+00:00

Hi all - I am having exact same issue on a three months old Surface Business Pro 11 w/ Windows 11 Pro. Installation rolls back at 38% with error code 0x800f0991. Performing an in‑place upgrade sounds like an enormous effort to me for trying to solve the issue of a buggy patch without really fixing the problem.
Norah-L 5,350 Reputation points Microsoft External Staff Moderator

2026-03-17T03:50:47.4433333+00:00
Hi Mark

I’m sorry to hear you’re experiencing the same issue!

Have you tried manually downloading and installing the update from the Microsoft Update Catalog?

Please also make sure drive C: has at least 10 GB of free space, then temporarily disable any third‑party antivirus software and try again.

If the issue continues, rename the software distribution folder. Open a Windows Command Prompt window, and then run the following set of commands (in sequence):

net stop wuauserv

rename c:\windows\SoftwareDistribution softwaredistribution.old

net start wuauserv

After that, press Win + R, type services.msc and restart the following services:

Windows Update (wuauserv), if it’s still running

Update Orchestrator Service (UsoSvc)

Background Intelligent Transfer Service (BITS) Hope one of these could help. Please let me know how it goes!

Best regards.
Norah-L 5,350 Reputation points Microsoft External Staff Moderator

2026-03-17T04:46:03.6133333+00:00

Good day Rob!

May I know if you have got any chance to check my answer?

I am glad to help if you have any other questions.
Mark 0 Reputation points

2026-03-17T07:41:52.2+00:00
Morning Norah, and thanks for response. Unfortunatey no workaround above has been successful. I disabled third-party antivirus software (Bitdefender) with all security features and ensured it will stay off. Even re-booted my computer after disabling.

Solution 1 - stand alone update:

Solution 2 - rename folder and re-starting services:

same issue with not having sucessful online update

tried again downloaded file
Norah-L 5,350 Reputation points Microsoft External Staff Moderator

2026-03-18T00:50:43.67+00:00

Good day Mark,

Thank you for posting back to let me know how it went!

In solution 1, the shown error message might indicate the KB you downloaded doesn’t match your system (either it’s for the wrong Windows version or the wrong architecture). For example, you may have downloaded an x64 package while your device is ARM64 (or vice versa).

Please try downloading it again following this: Press Win + R > type winver > and confirm your Windows edition and version. Then search for the KB number in the Microsoft Update Catalog and download the package that matches your exact Windows version and system architecture (for example, 22H2 x64).

You can check the Catalog section under How to get this update for details: March 10, 2026 — KB5079473 (OS Builds 26200.8037 and 26100.8037)

Hope this helps!

Yours sincerely.

Answer 5

Rob 35

How long should I wait for a fix?

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KB5079473 on build 26200.8037 fails to install - error code 0x800f0991

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