Windows Update Error 0x800f0845 Fix: Failed Installs, Rollbacks and Stuck Restarts

Short answer: Error 0x800f0845 is a Windows Update install failure, and the right fix depends on which phase failed: download, install, or the restart that finishes the update. Users report it with the October 2026 cumulative update KB5127648, but Microsoft does not list it as a known issue and documents no cause. Identify the phase first, then use the matching steps below.

Last reviewed: October 25, 2026.

What 0x800f0845 means when it appears with KB5127648

The code shows up when a cumulative update cannot be installed or committed. It does not say why. Microsoft has not documented a root cause for 0x800f0845 with this update, so any page claiming a single definite cause is guessing.

What we verified on 2026-10-25: Microsoft's KB5127648 page lists three known issues (Smart App Control may not turn on, Remote Desktop may fail for non-admin users with Administrator protection enabled, and some USB audio devices may fail to start or produce no sound). Error 0x800f0845 is not among them. Reports of the failure come from news sites and users, which we could not confirm against a Microsoft statement.

This page focuses on one pattern: the update that starts, then fails, rolls back, or hangs during the restart. If your problem is a different code, see 0x80070306 (a different failure path) or 0x800f0989, or the component-store error 0x80073712.

Step 1: Work out which phase failed

Windows applies a cumulative update in stages. A failure in each stage looks different and needs a different response.

What you saw	Likely phase	Go to
Stays at "Downloading" or restarts the download, error shown quickly	Download	Branch A
"Installing" reaches a percentage, then "Install error" with no restart	Install	Branch B
PC restarts, says "Undoing changes", then shows the old build and the error	Restart and commit (rollback)	Branch C
Restart screen stays on a percentage or spinning dots for a very long time	Restart (hang)	Branch D

Check Update history

Open Settings, Windows Update, Update history. Find KB5127648. "Failed to install" with the code means the attempt completed and failed. "Pending restart" means the install phase worked and the restart has not finished it. Note the date and time of the failure. You will need it for the log check.

Check Event Viewer

1. Press Start, type Event Viewer, and open it.
2. Go to Applications and Services Logs, Microsoft, Windows, WindowsUpdateClient, Operational. Look for Error or Warning entries at the failure time. Download-phase problems usually appear here first.
3. Go to Windows Logs, System and filter by source Servicing, or look in Windows Logs, Setup. Install and restart-phase problems tend to show up there.

Event text varies by PC, so read what your own entries say rather than expecting a match to this page. If you need more detail, Get-WindowsUpdateLog in PowerShell creates a readable WindowsUpdate.log on the desktop, and C:\Windows\Logs\CBS\CBS.log holds servicing detail.

Check the recovery options

Open Settings, System, Recovery. If you see Go back or Advanced startup, note them. You may need Advanced startup in Branches C and D.

Before you try anything risky

Back up important files first and create a restore point: press Start, type "Create a restore point", and follow the prompt for your system drive. A restore point does not replace a file backup, but it gives you a return path before you rename or reset anything. Do not disable Windows Security or antivirus as a fix.

Branch A: Fails at the download phase

If the update never fully downloads, the problem is usually the connection, storage or the update cache, not the install itself.

1. Check space. A practical rule of thumb is 20 GB free; this is not a Microsoft requirement for this update. Use Settings, System, Storage, Temporary files to clear Windows Update Cleanup and the Recycle Bin.
2. Switch the connection. Try a wired link or another network, and turn off a VPN while you test.
3. Run the Windows Update troubleshooter. Settings, System, Troubleshoot, Other troubleshooters.
4. Clear the update cache. After the restore point above, stop the update services and rename the SoftwareDistribution folder so Windows rebuilds it. The exact commands are in Fix 5 of the general guide.

Branch B: Fails during the install phase

Here the files arrived but could not be applied. This points toward servicing or file integrity, so check the system before retrying.

1. Restart once cleanly and make sure no other update is waiting for a restart.
2. Run the repair commands: DISM RestoreHealth, then SFC. Give RestoreHealth time. It can appear stuck at some percentages for a long time, so wait at least 30 minutes before deciding it hung. The commands and explanation are in Fix 6 of the general guide.
3. Read the Servicing and CBS entries from Step 1. If they name a specific different error, search for that code, it may have its own fix.
4. Install the package manually from the Microsoft Update Catalog (search KB5127648). Microsoft's KB page says Catalog installs need the latest servicing stack update and the KB5043080 checkpoint update installed first, in order. Skipping them can make the manual install fail too. Then run the file, with quotes if the path has spaces:

wusa.exe "C:\Users\YourName\Downloads\update-file.msu"

Leave out /quiet while you are diagnosing, so Windows can show you an error message. Download only from the Catalog, never from third-party sites.

Branch C: Restart, "Undoing changes", then back to the old build

This is the most frustrating pattern. The update installed in the offline restart phase, failed to commit, and Windows reverted. The rollback is Windows protecting you, so let it finish and do not power off during "Undoing changes".

1. Let the rollback complete and sign in. Confirm that your build is unchanged under Settings, System, About.
2. Remove peripherals before retrying: external drives, docks, extra monitors, smart card readers and printers. Retry with only keyboard, mouse and display connected.
3. Check for pending changes. Install any other waiting updates first, including driver updates under Windows Update, Advanced options, Optional updates, and restart.
4. Update firmware and drivers from the PC maker if they are old, especially storage and chipset drivers. Do not use unofficial driver packs.
5. Use the update's manual installer as in Branch B. Installing from a standalone file sometimes behaves differently from the Windows Update path.
6. Reinstall the current build over itself. Settings, System, Recovery, "Fix problems using Windows Update" re-installs the current build while keeping files and apps. It is Microsoft's own option. A repair install from the Windows 11 ISO (run setup.exe from the mounted ISO and choose Keep personal files and apps) follows the same idea.

Back up first. If the problem is persistent, note the log entries from Step 1 before you start, since they help any technician working on it.

Branch D: The restart hangs on a percentage

A long wait is not always a hang. Cumulative updates can take a long time on slower disks, and the percentage can sit still.

1. Wait. Allow at least 30 to 60 minutes if the disk light or fan shows activity. On a laptop, keep it plugged in.
2. If there is no activity for a long time, hold the power button to turn the PC off, wait 30 seconds, and start it again. Windows usually resumes or rolls back by itself. Forced power-off carries some risk, so use it only after waiting.
3. If it keeps looping, let Windows reach the recovery screen (it often appears after repeated failed starts). Choose Advanced options, then Troubleshoot, Advanced options, and try Startup Repair.
4. If the PC boots but is unstable after the update, go to Advanced options and use Uninstall Updates to remove the latest quality update, or System Restore to use the restore point.
5. Safe mode (Troubleshoot, Advanced options, Startup Settings, Restart) lets you check Update history and uninstall the update if Windows normally will not start.

If none of these gets a working desktop, stop and get help rather than trying more resets. Our remote repair and Windows support pages explain how a technician can work through it with you.

Fixes that apply to every branch

• Run the troubleshooter once. It is harmless.
• Run DISM and SFC once. They are documented, repair-only commands.
• Reset update components only after creating a restore point.
• Avoid registry edits unless a Microsoft page tells you to do them for your exact problem, and export the key first.

Common mistakes

• Cutting power during "Undoing changes" or "Working on updates".
• Pressing Retry many times without checking the phase that failed.
• Installing .msu files from unofficial sources.
• Deleting the SoftwareDistribution folder rather than renaming it.
• Skipping prerequisites for a manual install.
• Leaving the PC unpatched indefinitely. Try again after a few days if Microsoft publishes a newer update or a known-issue note.

Prevention

• Keep a practical 20 GB or so free on the system drive (a rule of thumb, not a Microsoft requirement for this update).
• Plug laptops in during updates and avoid long gaps between restarts.
• Keep a recent backup and restore point before large monthly updates.
• Check Microsoft's KB page and our October 2026 Patch Tuesday known issues if a monthly update looks unreliable.

Frequently asked questions

Why does 0x800f0845 appear after the restart and not before?

Some update steps run only during the offline restart phase. If one fails there, Windows rolls back and shows the error when you sign in. Event Viewer entries from that time help identify the step.

Is it safe to power off a PC stuck on a percentage?

Only as a last resort after a long wait. Windows normally recovers or rolls back on the next start, but a forced shutdown always carries some risk, so back up first when you can.

How can I tell if the install phase or the restart phase failed?

If the error appears without a restart, the install phase failed. If the PC restarted, said "Undoing changes" and returned to the old build, the restart phase failed.

Does Uninstall Updates delete my files?

It removes the latest quality update, not your personal files. Back up anyway before using recovery tools.

Why does the manual installer ask for KB5043080?

Microsoft's KB page says Catalog installs need the latest servicing stack update and the KB5043080 checkpoint update first, in order. Without them, a manual install can fail.

Is 0x800f0845 caused by KB5127648 itself?

That is not confirmed. As of 2026-10-25 Microsoft lists three other known issues for this update and none for this code.

Can a restore point fix a failed update?

It can return system settings and files to an earlier state if Windows boots in recovery, but it does not install the update. It is a safety net before repairs.

When should I stop and get help?

If the PC will not reach the desktop, loops through restarts, or you cannot back up your data, stop and ask a technician rather than trying more resets.