FIX KB5074109 Update Error 0x800f0991 in Windows 11 25H2/24H2
Here are the causes of the KB5074109 update failing to install with Error Code 0x800F0991 on Windows 11 25H2 or 24H2.This error is a servicing stack / component store failure, meaning Windows cannot correctly process or apply the update due to corrupted or missing system components.
Error 0x800F0991 indicates that Windows Update failed while processing system servicing operations.
It typically occurs when the update engine cannot validate or apply update packages.
⸻
Common Causes of KB5074109 Error 0x800F0991
1. Corrupted WinSxS Component Store (Primary Cause)
The WinSxS folder contains critical system components and manifests.
If it is:
corrupted
missing manifests (.mum/.cat)
out of sync
Windows cannot apply KB5074109 successfully.
⸻
2. Failed or Incomplete Previous Updates
If earlier cumulative or security updates:
failed
rolled back
were interrupted
they can leave the system in an inconsistent servicing state that blocks new updates.
⸻
3. Missing or Damaged Servicing Stack Update (SSU)
KB5074109 requires a fully functional Servicing Stack Update.
If the SSU is:
missing
outdated
corrupted
Windows Update cannot process the installation.
⸻
4. Corrupted Windows Update Cache
Damage in:
SoftwareDistribution
Catroot2
can cause Windows Update to mis-handle update metadata and fail during installation.
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