Windows 11- error 0x800f0991

Situatie

This is a Windows Update error that occurs when Windows fails to install an update — usually related to corrupted system files, update components, or the Windows image itself.

Solutie

Fix 1 — Run Windows Update Troubleshooter

Settings → System → Troubleshoot
→ Other troubleshooters
→ Windows Update → Run

Fix 2 — SFC Scan (System File Checker)

Run as Administrator:

cmd

sfc /scannow

Wait for it to complete, then restart and try updating again.

Fix 3 — DISM Repair

Run as Administrator in order:

cmd

# Check image health
DISM /Online /Cleanup-Image /CheckHealth

# Scan image health
DISM /Online /Cleanup-Image /ScanHealth

# Restore/repair image
DISM /Online /Cleanup-Image /RestoreHealth

After completion:

cmd

sfc /scannow

Then restart and try Windows Update again.

Fix 4 — Clear Windows Update Cache

Run as Administrator:

cmd

# Stop update services
net stop wuauserv
net stop cryptSvc
net stop bits
net stop msiserver

# Delete cache folders
rd /s /q C:\Windows\SoftwareDistribution
rd /s /q C:\Windows\System32\catroot2

# Restart services
net start wuauserv
net start cryptSvc
net start bits
net start msiserver

Then restart PC and try updating.

Fix 5 — Reset Windows Update Components

Run as Administrator:

cmd

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

regsvr32 atl.dll
regsvr32 urlmon.dll
regsvr32 mshtml.dll
regsvr32 shdocvw.dll
regsvr32 wuapi.dll
regsvr32 wuaueng.dll
regsvr32 wups.dll
regsvr32 wups2.dll
regsvr32 wucltui.dll

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

---

## Fix 6 — Check Disk Space

Windows 11 updates need at least **10–20GB free:**
```
Settings → System → Storage
or
This PC → check C: drive free space

Free up space:

cmd

# Disk Cleanup
cleanmgr

# Or Settings → System → Storage → Temporary files
```

---

## Fix 7 — Manually Install the Update
```
1. Note the KB number of the failing update
   (Settings → Windows Update → Update history)

2. Go to: https://catalog.update.microsoft.com

3. Search for the KB number

4. Download and install manually

Fix 8 — Check CBS Log for Exact Error

cmd

# View CBS log
notepad C:\Windows\Logs\CBS\CBS.log

# Or DISM log
notepad C:\Windows\Logs\DISM\dism.log
```
Search for **"error"** or **"failed"** to find the exact cause.

---

## Fix 9 — Disable Antivirus Temporarily

- Temporarily disable third-party antivirus
- Try the update again
- Re-enable after update completes

Windows Defender:
```
Windows Security → Virus & threat protection
→ Manage settings → Real-time protection → Off

Fix 10 — Windows Update via PowerShell

powershell

# Install Windows Update module
Install-Module PSWindowsUpdate -Force

# Check for updates
Get-WindowsUpdate

# Install all updates
Install-WindowsUpdate -AcceptAll -AutoReboot
```

---

## Fix 11 — In-Place Upgrade Repair

If nothing works — repair Windows without losing files:
```
1. Download Windows 11 ISO from microsoft.com
2. Mount the ISO
3. Run setup.exe
4. Choose "Keep personal files and apps"
5. This repairs Windows while preserving data

Fix 12 — Check BCD / Boot Configuration

Run as Administrator:

cmd

bootrec /fixmbr
bootrec /fixboot
bootrec /rebuildbcd

Tip solutie

Permanent

Follow Us