Use sfc /scannow to Fix Corrupted System Files

What sfc /scannow Actually Does

The System File Checker scans every protected system file on your Windows installation, compares each one against a known-good version stored in the Windows component store (C:\Windows\WinSxS), and automatically replaces any file that's missing, corrupted, or has been improperly modified. Corrupted system files cause a wide range of symptoms — random crashes, apps that won't launch, Windows Update failures, blue screens with no obvious driver at fault — and SFC is the first tool to reach for whenever the cause is unclear but system-level corruption is suspected.

Step 1: Open an Elevated Command Prompt

1. Press Win, type cmd.
2. Right-click Command Prompt and select Run as administrator. SFC will refuse to run without elevated privileges.

Step 2: Run the Scan

sfc /scannow

The scan reaches 100% in anywhere from 10 to 30 minutes depending on drive speed. Don't close the window or interrupt it — let it finish.

Possible Results

• "Windows Resource Protection did not find any integrity violations" — no corruption found. If you're still having the original problem, the cause lies elsewhere.
• "Windows Resource Protection found corrupt files and successfully repaired them" — it fixed the problem. Restart and retest.
• "Windows Resource Protection found corrupt files but was unable to fix some of them" — this is where DISM comes in (next section).

Step 3: If SFC Can't Fix Everything, Run DISM First

SFC repairs files by pulling replacements from your local component store, but if that store is itself corrupted, SFC has nothing valid to copy from and fails. DISM repairs the component store using Windows Update as its source, which then lets a follow-up SFC scan succeed.

DISM /Online /Cleanup-Image /RestoreHealth

This requires an active internet connection, since it pulls replacement files from Windows Update by default. It typically takes 10-20 minutes and can appear to stall at certain percentages (particularly 20% and 40%) — this is normal; let it run.

If DISM Can't Reach Windows Update

On a machine with no internet access, or where Windows Update itself is broken, you can point DISM at a Windows installation image instead. Mount an ISO of the same Windows version and build, then run:

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

Adjust the drive letter (D:) to match where the ISO is mounted, and confirm the index number (:1) matches the edition you're running — check available indexes with DISM /Get-WimInfo /WimFile:D:\sources\install.wim first.

Step 4: Run SFC Again

sfc /scannow

With a repaired component store behind it, this second pass should now succeed in replacing the files it couldn't reach before.

Step 5: Restart and Confirm

Reboot the PC and check that the original symptom is resolved. Some replaced system files only take effect after a restart, particularly ones in use by the running OS session.

Checking the Detailed Log

SFC writes detailed results to CBS.log. To pull out just the SFC-related lines into a readable file on your desktop:

findstr /c:"[SR]" %windir%\Logs\CBS\CBS.log > "%userprofile%\Desktop\sfcdetails.txt"

Open sfcdetails.txt in Notepad to see exactly which files were flagged and whether each was successfully repaired — useful if you need to identify a specific problem file rather than just knowing "something" was fixed.

When SFC and DISM Don't Solve It

• Corruption keeps coming back after repeated scans: a failing hard drive or RAM can corrupt files as fast as SFC replaces them. Run chkdsk /f on the drive and Windows Memory Diagnostic to rule out hardware.
• DISM reports it also can't fix the image: at this point an in-place upgrade repair (reinstalling Windows over itself while keeping files and apps, via Settings > Windows Update > or booting from installation media and choosing "Keep personal files and apps") is the next step up.
• Problem is isolated to one specific app, not system-wide: SFC only protects core Windows system files, not third-party application files — reinstalling the affected app is the right fix instead.

Quick Reference

sfc /scannow
DISM /Online /Cleanup-Image /RestoreHealth
sfc /scannow
findstr /c:"[SR]" %windir%\Logs\CBS\CBS.log > "%userprofile%\Desktop\sfcdetails.txt"