KB5129195 Install Error 0x800f0993: How to Fix the September 2026 Windows Update

What is KB5129195, and why does it fail with 0x800f0993?

KB5129195 is the out-of-band (emergency) cumulative update Microsoft released on September 15, 2026 for Windows 11 versions 24H2 and 25H2 to repair problems introduced by the September 8 security update, KB5124008. When it shows "Install error - 0x800f0993" in Windows Update history, the download usually worked but Windows could not apply the package, most often because of a servicing store problem, a leftover pending update, or a conflict with a partly installed earlier patch. The error text on your screen is generic. It tells you the installation step failed, not the exact cause.

If you landed here after copying a line such as "2026-09 Security Update (KB5129195) (26200.9457) Install error - 0x800f0993" from Settings, Windows Update, Update history, you are in good company. This pattern repeats every month: Microsoft ships a new KB number, a small share of PCs fail to install it, and the Update history entry shows the KB number and a hex code. The KB number changes. The troubleshooting steps do not change much. This guide gives you the full sequence in the order that works fastest, then explains how to read the same kind of error next month, when the KB number will be different.

Two honest notes before we start. First, Microsoft has not published a page saying that KB5129195 specifically triggers 0x800f0993; the code is a general servicing failure that appears across many updates, and our advice is based on how that code behaves in practice. Second, if your PC is working normally and only the history shows a red entry, you may not need to do anything at all. We explain how to tell in the section on whether the failed install even matters.

The three September 2026 updates in plain English

Three KB numbers dominate September 2026: KB5124008 is the regular Patch Tuesday update of September 8, KB5129195 is the emergency fix of September 15 that mostly repairs it, and KB5124010 is the optional preview update of September 22 that also addresses a File History backup bug. Knowing which one you need saves hours, because people often try to fix the wrong package.

Builds and dates above come from Windows Latest coverage and Microsoft's own support pages as described in our sources list. Details such as the exact build for Windows 11 version 24H2 versus 25H2 follow the usual pattern where both versions share the same servicing branch and differ only in the first part of the build number.

The simplest way to read this table: KB5124008 started the trouble, KB5129195 is the cumulative fix that replaces it, and KB5124010 is a bonus for people hit by the backup bug. If KB5129195 refuses to install, you are not choosing between three updates. You are trying to get the newest cumulative update onto a system that has trouble accepting it.

Timeline: what happened and when

The September 2026 sequence ran from the Patch Tuesday release on September 8, through Microsoft's acknowledgement of failures around September 12, to the emergency KB5129195 on September 15 and the optional KB5124010 on September 22. Seeing the order helps you decide what to do, because an install error that began on September 9 likely involves a different failure than one that started after you tried to catch up on September 20.

• September 8: Microsoft releases KB5124008 as the monthly security update for Windows 11 24H2 and 25H2.
• September 12 (approximately): Press coverage reports that Microsoft admitted the update was breaking several features, including Remote Desktop Services connections, folder sharing between Windows and Linux virtual machines, and the desktop in virtual-desktop scenarios.
• September 15: KB5129195 appears as a mandatory out-of-band cumulative update. Reported install time is around 10 minutes, with more than one restart possible if Secure Boot related updates are pending on the device.
• September 22: The optional update KB5124010 follows, with the File History fix described by Windows Latest as partial.
• September 23: Microsoft confirms that File History and similar backup tools could stop backing up files because of a bug in the September update.
• October 13, 2026: The next Patch Tuesday, which should bring the October cumulative update and, per the preview notes, the fixes previewed in KB5124010.

Not everything is fixed. Windows Latest reported that KB5129195 does not address the AMD graphics black screen and freeze complaints some users link to the September updates. If your symptom is a black screen rather than an install error, that is a separate problem, and the install-error steps in this guide will not cure it.

Does the failed install actually matter?

Yes and no. If KB5124008 installed successfully and your build number is already 26200.9445 or higher, you still hold the September security fixes, and the failed KB5129195 only means you are missing the repairs for the specific feature bugs. If neither installed, you are missing a month of security fixes, which is the more serious case. Check with winver (Windows key plus R, type winver, Enter). Compare the last digits of the build against the table above. That one check decides how urgent the rest of this page is for you.

Windows Update will normally retry on its own, often with the same result. That is why a failed entry can sit in the history for days, then fail again after the next restart. Our broader guide on Windows Update stuck or failing covers the cases where the progress bar itself freezes. This page focuses on the install error that appears after the download finishes.

Which error codes show up with these updates

Five codes account for most failed monthly updates: 0x800f0993 (the package could not be applied), 0x80073712 (a needed servicing file is missing or damaged), 0x800f0922 (the install ran out of room or could not finish on the system or recovery partition), 0x80070002 (a file could not be found) and 0xc0430001 (a boot-time failure after the update). They look similar in Update history, but each points at a different layer, so matching your code to the right fix saves time.

The meanings above are practical summaries, not exact Microsoft definitions. Microsoft does not publish a single official description for every servicing code, and the same code can come from several root causes. Use the table to choose a direction, then confirm by whether the fix works.

A useful rule: if your error is 0x800f0993 or 0x80073712, the problem is almost always inside Windows' own component store, which DISM can repair. If it is 0x800f0922, look at disk space and network first. If it is 0xc0430001, the PC fails to start and you must work from the recovery environment, which is a different set of steps covered later.

Fixes in order: start with the five-minute ones

Work through the fixes in this order: restart and retry, run the Windows Update troubleshooter, free disk space and disconnect VPN or extra devices, install the update manually from the Microsoft Update Catalog, then repair with DISM and sfc, and only then reset the update components. Each step is safer and faster than the next one, and most people stop within the first four.

Step 1: restart twice and retry

A first restart finishes any pending operation left from the earlier attempt, such as a half-applied KB5124008 or a Secure Boot related step. The second restart gives Windows a clean start before the new try. After the second restart, open Settings, Windows Update, and select Check for updates. If KB5129195 is listed as "Pending restart" or "Downloading", leave the PC alone on power for 30 minutes before judging it failed.

Step 2: run the Windows Update troubleshooter

Open Settings, System, Troubleshoot, Other troubleshooters, and run Windows Update. In some Windows 11 builds the same tool is also reachable through Get Help. The troubleshooter restarts services, clears some stale cache entries and fixes simple registry flags. It rarely solves 0x800f0993 alone, but it is cheap and sometimes unlocks a stuck pending state. Restart again, even if it says nothing was found.

Step 3: free space and remove interference

Make sure the C: drive has at least 20 GB free. Cumulative updates need room for the package, the rollback copy and temporary files. Open Settings, System, Storage, and use Temporary files to remove the Windows Update Cleanup, Delivery Optimization files and the Recycle Bin. Then unplug docks, printers, external drives and USB audio gear. Given that September's problems involved USB audio and external drives, removing them also eliminates one possible source of confusion. Disconnect any VPN and disable third-party security suites temporarily; keep Microsoft Defender on.

Step 4: confirm the date, time and region

An incorrect clock or a mismatch between the region and the time zone sometimes breaks the secure connection to Microsoft's update servers and shows as a servicing error. Set Settings, Time and language, Date and time to automatic, then press Sync now.

If the retry still fails with 0x800f0993, move on to a manual install. It skips the Windows Update client entirely, which is often where the problem lives. If the full steps below feel like too much, call 1 (888) 711-9428 and a technician can run them remotely while you watch.

Install KB5129195 manually from the Microsoft Update Catalog

The Microsoft Update Catalog lets you download the standalone package (an .msu file) for KB5129195 and install it yourself, bypassing the Windows Update client. This works when the automatic path keeps failing with 0x800f0993, because the installer reads the package directly instead of relying on the update service state. It does not fix a corrupted component store, but it often succeeds where the automatic path loops.

1. Find your build and architecture. Press Windows plus R, type winver, and note the version (24H2 or 25H2). Open Settings, System, About, and check "System type": 64-bit x64 for most PCs, ARM64 for Copilot+ PCs with Snapdragon chips.
2. Open the Catalog. In your browser go to catalog.update.microsoft.com and search for KB5129195.
3. Pick the right row. Choose the entry for Windows 11 that matches your version and architecture. Rows mention "x64-based" or "arm64-based". Do not install a package for a different architecture.
4. Download the .msu file. Click Download, then the link in the pop-up. Save it to a simple folder such as C:\Temp. Files are large (often over 1 GB for cumulative updates).
5. Install it. Double-click the .msu and approve. Alternatively open Terminal as administrator and run wusa.exe C:\Temp\yourfile.msu, using the actual file name.
6. Restart when asked, even if it says it is optional. Check the build number again with winver.

What if the manual install also fails?

If the .msu install stops with the same code or a new one, the problem is in the Windows servicing stack or the component store, not in the delivery channel. That is the cue to repair Windows with DISM and sfc. Some people also hit error 0x80070003 or 0x800f0922 at this stage; the repair steps are the same. A second possibility is that a partly installed older update is blocking the new one, which we cover in the pause and uninstall section.

Note that the catalog may list more than one package for the same KB number on some months. When you see two similar rows, the later date and the larger file normally contain the full cumulative update, but if you are unsure, install the one that exactly matches your Windows version string. Skipping an unnecessary checkpoint package does no harm.

Repair Windows with DISM and sfc (the most reliable fix for 0x800f0993)

DISM repairs the Windows component store, the library that update packages build from, and sfc then checks the protected system files against that store. Running them in the right order, DISM first and sfc second, fixes the large majority of 0x800f0993 and 0x80073712 failures. The whole sequence takes 20 to 40 minutes and needs an internet connection.

1. Click Start, type cmd, right-click Command Prompt, choose Run as administrator.
2. Run: DISM /Online /Cleanup-Image /CheckHealth to see whether corruption has been flagged. It takes seconds.
3. Run: DISM /Online /Cleanup-Image /ScanHealth. This scan takes several minutes and may appear to sit at 20 percent or 62 percent. That is normal.
4. Run: DISM /Online /Cleanup-Image /RestoreHealth. This downloads clean files from Windows Update and repairs the store. Wait for "The restore operation completed successfully".
5. Run: sfc /scannow. Wait for the verification to reach 100 percent. A message such as "Windows Resource Protection found corrupt files and successfully repaired them" is the good outcome.
6. Restart the PC and try Windows Update again, or install the manual package.

When DISM RestoreHealth itself fails

If DISM stops with 0x800f081f, 0x800f0906 or 0x800f0954, it could not find clean source files online. Use a Windows ISO as the source. Download the Windows 11 ISO that matches your version from Microsoft, double-click to mount it, and note the new drive letter (for example E:). Then run:

DISM /Online /Cleanup-Image /RestoreHealth /Source:WIM:E:\sources\install.wim:1 /LimitAccess

Some ISOs contain install.esd instead of install.wim; in that case replace the path accordingly with ESD:E:\sources\install.esd:1. The trailing number is the image index; index 1 is often Home or the first edition, so if it fails, run DISM /Get-WimInfo /WimFile:E:\sources\install.wim to list the indexes and pick the one that matches your edition. The ISO must match your Windows language and be the same version or newer. Our page on error 0x800f081f goes deeper into this case.

Reading the logs when you want to know why

Windows writes servicing details to C:\Windows\Logs\CBS\CBS.log and to C:\Windows\Logs\DISM\dism.log. Open CBS.log in Notepad and search for the word "Error" or for the code. Lines near the match name the package or file that failed. You do not need to understand every line. Look for a KB number or a component name that appears repeatedly, then search for it. A repeated mention of a driver package, for example, hints that a driver store entry is the culprit, and a repeated mention of "pending.xml" points at the pending-operations fix below.

After DISM and sfc complete cleanly, restart and retry. In our experience, a repaired component store is the single most common difference between "fails every time" and "installs without drama". If it still fails, the next layer is the Windows Update cache and services.

Reset the Windows Update components

Resetting the update components stops the update services, renames the two cache folders (SoftwareDistribution and catroot2) so Windows rebuilds them fresh, and restarts the services. It clears corrupted downloads and stale metadata that cause 0x80070002 and often help with 0x800f0993, and it is safe because Windows recreates the folders automatically.

Open Command Prompt as administrator and run these commands one at a time, in this order:

1. net stop wuauserv
2. net stop cryptSvc
3. net stop bits
4. net stop msiserver
5. ren C:\Windows\SoftwareDistribution SoftwareDistribution.old
6. ren C:\Windows\System32\catroot2 catroot2.old
7. net start wuauserv
8. net start cryptSvc
9. net start bits
10. net start msiserver

Restart the PC, open Windows Update and check for updates. The first scan takes longer than normal because the history and metadata are rebuilt. Your update history list may look shorter afterwards; that is expected. If a "Access is denied" error appears when renaming, a service did not stop. Run net stop wuauserv again and check that no Windows Update window is open.

Check the services are set correctly

Press Windows plus R, type services.msc, and confirm that Windows Update is set to Manual (Trigger Start), Background Intelligent Transfer Service to Manual, Cryptographic Services to Automatic, and Windows Modules Installer to Manual. Third-party "debloat" or privacy tools sometimes disable these. If one is Disabled, set it to its normal startup type and start it. Tools that edit the registry for privacy are a frequent unseen cause of monthly update failures.

Pause, uninstall or roll back: when it is the right move

Pausing updates buys time and uninstalling a bad update removes it, but both leave you without the latest security fixes, so use them only for a specific reason: a work-critical feature broke after an update, or the PC will not complete the install after the repairs above. Do not leave a PC paused for weeks. The September 8 security update contained real fixes, and KB5129195 is meant to replace it, so skipping both leaves a gap.

Uninstall KB5124008 (and then try KB5129195)

If KB5124008 installed but misbehaves, and KB5129195 will not apply on top of it, removing KB5124008 and installing the emergency package from the catalog can sometimes work, because the cumulative package is meant to supersede it. Open Settings, Windows Update, Update history, scroll to Uninstall updates, find the September 2026 security update and select Uninstall. Restart. Then install KB5129195 manually.

From a command line you can use wusa /uninstall /kb:5124008. Some cumulative updates cannot be removed this way because they include servicing stack changes. In that case Windows reports that the update is not installed or cannot be removed. Do not force it with third-party tools.

Pause updates for a week

Settings, Windows Update, Pause updates, choose 1 week. A short pause is reasonable when you must finish a presentation, a tax filing or payroll before dealing with a failing update. Put a reminder on your calendar. Our related guide on Windows that will not boot after an update covers the worse case where an update leaves the PC unable to start.

If the PC will not start: the recovery route

If you see a blue screen with 0xc0430001 or a restart loop after the update, hold the power button for 10 seconds to turn the PC off, repeat until Windows shows the recovery menu, then choose Troubleshoot, Advanced options, Uninstall Updates, and remove the latest quality update. Have the BitLocker recovery key ready, because recovery steps can prompt for it. The page on 0xc0430001 boot failure walks through the full recovery sequence.

When to simply wait

Wait if your PC works normally, the failure is the only symptom, and Windows Update retries on its own: the next cumulative update on Patch Tuesday (October 13, 2026) replaces KB5129195 and often installs cleanly because it ships with a newer servicing stack. A failing install is not an emergency by itself. A PC that is stable, patched through KB5124008, and protected by Microsoft Defender is in a reasonable place for a week or two.

• Wait if build already 26200.9445 or higher, no symptoms, personal or non-critical use.
• Act today if the PC is a remote desktop host, a shared office PC, or reports the symptoms KB5129195 was meant to fix, such as dropped Remote Desktop sessions or lost USB audio.
• Act today if neither KB5124008 nor KB5129195 is installed and the PC is internet-facing.
• Do not wait if the PC fails to boot, blue screens, or shows BitLocker recovery prompts after the attempt.

Waiting is not the same as ignoring. Set a reminder for October 14, one day after Patch Tuesday, to check Update history again. If October's update also fails with the same code, the problem is in your PC, not in September's package, and the DISM, sfc and reset steps above become mandatory. If you rely on File History, do not wait to verify that your backups still run; see the note in the next sections about backups after the September bug.

Last resort: in-place repair upgrade

When everything above fails, an in-place repair reinstalls Windows over itself and keeps your files, apps and settings. Download the Windows 11 ISO from Microsoft that matches your current version, mount it, run setup.exe and choose to keep personal files and apps. It takes 45 to 90 minutes. Back up first, suspend BitLocker, and plug in the laptop. This replaces every system file and nearly always clears persistent component store damage. If you prefer not to do it alone, a remote technician can run it with you; see the call-a-pro section.

The monthly pattern: a new KB every Patch Tuesday and how to read the error

Microsoft releases cumulative security updates on the second Tuesday of each month (Patch Tuesday), each with a new KB number and build, and an optional preview update near the end of the month. The error you see in Update history always follows the same format: year-month, update type, KB number, build, then "Install error" and a hex code. The KB number tells you which month failed, the hex code tells you which layer failed. Once you can read that line, you can fix next month's failure without waiting for a new article.

Take the line "2026-09 Security Update (KB5129195) (26200.9457) Install error - 0x800f0993" and split it:

A repeatable monthly routine

1. On Patch Tuesday, wait one to three days before you install if the PC is business critical. Early adopters find the problems first, as September 2026 showed.
2. Make sure you have a restore point or backup, and 20 GB free.
3. Install, restart, and run winver to confirm the new build.
4. If Update history shows "Install error", copy the exact line. Search the KB number plus the code.
5. Apply the same ladder: retry, troubleshooter, space, manual install, DISM and sfc, component reset.
6. Only after those, consider uninstall, pause or in-place repair.

Previous months followed the same shape. Our April 2026 Patch Tuesday guide and July 2026 Patch Tuesday guide cover earlier cycles. Search data on our own pages also shows people pasting update lines for other months, including earlier KB numbers, into search with the same 0x800f0993 code. The cause was never the KB itself; it was the PC's ability to apply any cumulative update.

Why a few PCs fail while most succeed

A cumulative update is applied against a very large store of system components. Failures cluster on machines with a history of interrupted updates, forced shutdowns during "Working on updates", disk errors, aggressive cleaners that delete files from WinSxS, and third-party driver or security tools that hook the servicing process. A healthy PC takes the package without a second thought. A PC with a damaged store may succeed for months, then fail the moment a package touches the damaged component. That is why you may see "it worked last month, now it fails".

September 2026 symptoms and what to do about each

The September 2026 updates were linked to Remote Desktop Services failures, USB multichannel audio problems, folder sharing between Windows and Linux virtual machines, File History backups that stopped running, and, for some AMD graphics users, black screens. Which fix applies depends on your symptom: KB5129195 targets the first group, KB5124010 targets File History, and the AMD complaints were not addressed by KB5129195.

Remote Desktop or Remote Desktop Services

If you connect to a PC or server over RDP and sessions drop or fail, the emergency update was reported to fix Remote Desktop Services stability. For small offices that rely on RDP, this is the reason to prioritize KB5129195 over waiting. Test one connection from another PC after installing. Do not expose RDP to the internet; use a VPN or a remote support tool.

USB audio and headsets

Complaints included multichannel (8-channel and 3D) audio on USB devices. The emergency update was reported to restore this, though some users still reported complete loss of audio on certain USB devices afterwards. As a stopgap, try a different USB port, update the device driver from the manufacturer, or use Bluetooth or the 3.5 mm jack until a driver update arrives.

File History and backups

Microsoft confirmed on September 23 that File History could stop backing up files because of a bug introduced in the September update. Windows Latest described the later optional update, KB5124010, as only a partial fix. Symptoms: a prompt saying "Reconnect your drive" even when the external drive is connected, or backups that quietly stop. Do not assume your backups are working. Open Control Panel, File History, and check the "Files last copied" date. Make a manual copy of critical folders to an external drive, and keep a second copy in the cloud. Our guide on cloud backup for Canadian businesses helps you set up a second layer, and data recovery after a Windows update or reinstall explains what to do if files already vanished.

Folder sharing with WSL, Hyper-V and virtual machines

Developers who mount Windows folders inside a Linux virtual machine using the Plan9 protocol saw failures after KB5124008. KB5129195 was reported to address them. If you rely on this setup, install the emergency update before your next work session and test a file write.

Black screens on AMD graphics

Windows Latest reported that KB5129195 does not fix black screens and freezes on several Radeon models, and that Microsoft had not acknowledged them as a known issue at the time. If that is your symptom, do not keep reinstalling updates. Update the AMD driver from the manufacturer, run sfc /scannow, and consider uninstalling the September update only if the problem began right after it. The wider rule is to match the symptom to the fix instead of installing every package in the hope that one works.

Domain-joined and business PCs

Some reports mention domain trust and Credential Guard warnings associated with the September cycle. We could not confirm the scope on a Microsoft page, so treat it as unconfirmed. If you manage a domain, read the Windows release health dashboard for the current status before deploying to all machines, and pilot on a few PCs first.

Three realistic scenarios

The three examples below show how the same error resolves at different levels of effort: a quick retry for a healthy home laptop, a component repair for a small office PC, and a recovery route for a PC that stopped booting. They are illustrative examples based on common situations, not specific clients.

What it costs: do it yourself or ask a technician (CAD)

Fixing a failed Windows update yourself costs nothing but time, usually 30 to 90 minutes. An expert remote consultation with IT Cares is 119.99$ CAD for 60 minutes, which covers diagnosis, the repair steps in this guide and a check that updates install afterwards. The risk of waiting is not a bill but a gap in security fixes and backups.

If you add the cost of lost work, the numbers shift. An office of six people who cannot use their file server for half a day loses more than 120$ of productive time several times over. The value of a technician is mostly the avoided downtime and the confidence that backups work.

Prevent the next failure

Keep at least 20 GB free, shut down properly, avoid registry cleaners, install updates when the PC is on power and idle, and keep a verified backup. These five habits prevent most monthly update failures.

Official resources

For the authoritative fix lists, known issues and downloads, use Microsoft's own pages rather than third-party sites. Windows release health lists active and resolved known issues for each release, the Microsoft Support page for each KB lists the fixes and the manual download link, and the Update Catalog hosts the standalone packages. The sources list at the end of this guide links to the pages we used.

For Canadian small businesses, the Canadian Centre for Cyber Security recommends applying security updates promptly and keeping tested backups, which is consistent with the advice here. In Quebec, businesses covered by Law 25 are expected to protect personal information with reasonable safeguards, and unpatched systems or failed backups can weaken that position.

When to call a professional

Call a technician if the PC does not boot after an update, if the same error returns after DISM, sfc and the component reset, if the PC holds data without a backup, or if it is a business machine that people depend on. Stopping early costs less than a damaged installation.

IT Cares has provided remote and on-site IT support in Quebec and across Canada since 2014. A remote session lets a technician connect while you watch, repair the update components, install the missing updates, and verify that File History or your other backups are running. If files appear to be missing, we advise stopping and calling before running more tools.

To talk to someone now, call 1 (888) 711-9428 or book a remote session. Tell us the exact line from Update history, your build number and what you already tried.

Frequently asked questions

Related guides

• Windows error 0x800f0993
• Fix error 0x80073712
• Fix Windows 11 update error 0x800f0922
• Fix error 0x80070002
• Fix Windows 11 boot failure 0xc0430001
• Fix error 0x800f081f
• Windows Update stuck or failing
• Windows will not boot after an update
• Data recovery after a Windows update
• April 2026 Patch Tuesday guide

Sources and official references

Last verified: October 1, 2026

• Windows Latest: KB5129195 emergency update (Sept 15, 2026)
• Windows Latest: September update problems (Sept 12, 2026)
• Windows Latest: File History bug confirmed (Sept 23, 2026)
• Microsoft Support: KB5124008 release notes
• Microsoft Windows release health
• Microsoft Update Catalog