What System File Checker does and when you need it

System File Checker (SFC) is a Windows tool that scans your computer for corrupted system files and replaces them with working copies. You run it from the command line, and it works in the background — you do not need to read anything or pay for it. It comes built into Windows.

You need it when Windows behaves oddly in ways that restart or updates do not fix: programs crash without reason, you see error messages about missing files, Windows Update fails repeatedly, or your computer is slow in ways that have no obvious cause. SFC will not solve every problem, but it catches the ones caused by files that got damaged or deleted by accident, by a failed update, or by malware.

SFC takes 15 to 60 minutes depending on your hard drive speed and how many files it needs to check. You should close other programs before you start, but you can use your computer while it runs — it does not lock you out.

Key Takeaways

  • System File Checker runs from Command Prompt with administrator access, which you open by right-clicking and selecting "Run as administrator".
  • The command you type is sfc /scannow, and Windows will scan all protected system files and repair any it finds corrupted.
  • If SFC finds problems but cannot fix them, it will tell you so in the results — you may need to run it again or restart your computer first.
  • You should restart your computer before running SFC so that Windows is not actively using the files it needs to check.

How to open Command Prompt with the right permissions

System File Checker requires administrator access, which means Windows needs your permission to make changes to system files. Opening Command Prompt the normal way will not work — you have to tell Windows you want the elevated version.

On Windows 10 or 11, click the Start button and type cmd into the search box. You will see "Command Prompt" appear in the results. Right-click on it and select Run as administrator. Windows will ask "Do you want to allow this app to make changes to your device?" — click Yes.

You will now see a black window with white text that says something like C:\Windows\system32> at the bottom. This is the command line, and it is ready for your next instruction. If you see a window that says "Access Denied" or does not show the administrator path, close it and try again — you may have clicked the wrong option.

Running the scan and what to expect

Type the following command exactly as it appears: sfc /scannow

Press Enter. Windows will begin scanning when ready. You will see a progress bar that says "Scanning system files" with a percentage. This is normal and expected. Do not close the window or restart your computer while the scan is running — let it finish completely, even if it takes an hour.

When the scan finishes, you will see one of three messages at the bottom of the window. The most common is "Windows Resource Protection did not find any integrity violations" — this means your system files are fine and SFC found nothing to repair. If you see this, your problem lies elsewhere and you should look at other causes like disk space, malware, or hardware failure.

When SFC finds problems and repairs them

If SFC finds corrupted files, it will say "Windows Resource Protection found corrupt files and successfully repaired them." This is good news — the tool fixed the problem. You should restart your computer so the repaired files take effect. Close Command Prompt and restart normally.

After restart, test whether your original problem is gone. If programs still crash or you still see errors, run SFC again — sometimes multiple passes are needed, or the problem was caused by something else that SFC cannot fix.

Occasionally SFC will say "Windows Resource Protection found corrupt files but was unable to fix some of them." This means some files are too damaged or locked for SFC to repair on its own. Restart your computer and run SFC again — Windows may be able to fix them the second time when they are not in use. If it still cannot fix them after a second run, you may need to use the Deployment Image Servicing and Management tool (DISM), which is a more powerful repair tool, or contact Microsoft Support.

Restart your computer before running SFC

System File Checker works best when Windows is not actively using the files it needs to check. Before you open Command Prompt, restart your computer. This clears out temporary files and makes sure Windows is not holding locks on the system files SFC needs to scan.

You do not need to restart between multiple SFC runs if you are running it more than once in the same session — only restart before the first scan and after SFC reports it found and repaired files.

When SFC is not the right tool

System File Checker only repairs Windows system files, not your personal files, program files, or data. If you are missing photos, documents, or files you created, SFC will not help — you need file recovery software or a backup instead.

SFC also will not fix problems caused by malware, insufficient disk space, failing hard drives, or hardware issues. If your computer is running slowly, SFC may help if the slowness is caused by corrupted system files, but it will not speed up an old computer or one with a full hard drive. If you suspect malware, run a full antivirus scan before or after SFC — the two tools work on different problems.

Frequently Asked Questions

Do I need to close all my programs before running SFC?

You should close programs that are not essential, especially anything that uses a lot of memory or disk access. SFC will run while other programs are open, but it works faster and more reliably when your computer is not busy. Closing your web browser, email, and other applications is a good idea.

Can I use my computer while SFC is running?

Yes, you can use your computer normally while the scan runs in the background. The scan will take longer if you are actively using programs, but it will not damage anything. Just do not restart or shut down your computer while the scan is in progress.

What if I see "Access Denied" when I try to open Command Prompt as administrator?

This usually means your user account does not have administrator permissions. If you are on a work computer, contact your IT department. On a personal computer, you may need to change your account type in Settings under "Accounts" to Administrator, or create a new administrator account.

Should I run SFC regularly as maintenance?

No — SFC is a repair tool, not a maintenance tool. Run it only when you notice problems like crashes, error messages, or Windows Update failures. Running it regularly will not prevent problems and wastes time. A healthy Windows installation rarely needs SFC.

What is the difference between SFC and DISM?

SFC scans and repairs Windows system files on your computer. DISM is a more powerful tool that can repair the Windows image itself and is used when SFC cannot fix problems. You typically try SFC first, and only use DISM if SFC fails twice.