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 understand how it works, only when to use it and what to expect while it runs.

Use System File Checker when your computer is behaving oddly in ways that suggest a file corruption problem: programs crashing without an error message, Windows features not loading, or your computer refusing to shut down or restart properly. It is also worth running if you have recently had a power failure, a forced shutdown, or a failed Windows update, since those can leave files in a broken state.

System File Checker will not speed up a slow computer, fix a virus, or recover deleted files. If your problem is a missing program or a file you accidentally deleted, this tool will not help. But if Windows itself has a damaged file, System File Checker can often fix it without you having to reinstall Windows.

Key Takeaways

  • System File Checker runs from Command Prompt with administrator access, using the command sfc /scannow, and takes 15 to 60 minutes depending on your drive size.
  • You must restart your computer before running the scan, and you should close all open programs and browser windows first.
  • If System File Checker finds problems, it will repair them automatically — you do not need to do anything while the scan runs.
  • After the scan finishes, read the summary message to see whether files were found and repaired, and restart your computer if prompted.
  • If the scan finds problems but cannot repair them, or if your problem persists after repair, you may need to run the scan again or contact someone who can help troubleshoot further.

Opening Command Prompt with administrator access

System File Checker requires administrator permissions to run, which means you cannot launch it from the regular Command Prompt. You must open Command Prompt as an administrator.

On Windows 10 or 11, click the Start button (the Windows logo in the bottom left corner). Type cmd into the search box. Right-click on "Command Prompt" when it appears in the results, and select "Run as administrator" from the menu. Windows will ask you to confirm that you want to allow this program to make changes to your computer — click "Yes".

If you see "Command Prompt (Admin)" in the title bar at the top of the window, you have opened it correctly. The window will have a black background and white text.

Running the System File Checker scan

Once Command Prompt is open with administrator access, type the following command exactly as it appears: sfc /scannow (that is the letters s, f, c, a space, a forward slash, and the word scannow). Press Enter on your keyboard.

The scan will begin when ready. You will see a progress bar that says "Scanning system files" and a percentage that increases as the scan runs. Do not close the Command Prompt window, restart your computer, or unplug your computer while the scan is running. The scan typically takes 15 to 60 minutes depending on how large your drive is and how many files Windows needs to check.

While the scan runs, you can use your computer normally — open other programs, browse the internet, or work on documents. The scan runs in the background and will not prevent you from doing other things, though your computer may run slightly slower than usual.

Understanding the scan results

When the scan finishes, Command Prompt will display a message that tells you what happened. The message will be one of three types.

If the message says "Windows Resource Protection did not find any integrity violations," the scan found no corrupted files. Your problem may be caused by something other than file corruption — a virus, a failing hard drive, or a software conflict. You can close Command Prompt and try a different troubleshooting step.

If the message says "Windows Resource Protection found corrupt files and successfully repaired them," the scan found problems and fixed them automatically. Close Command Prompt and restart your computer. After the restart, test whether your original problem is resolved. Many issues disappear after System File Checker repairs the files.

If the message says "Windows Resource Protection found corrupt files but was unable to fix some of them," the scan found problems but could not repair all of them. This usually means the corrupted files are in use by Windows at that moment, or the files are protected in a way that prevents repair. Close Command Prompt, restart your computer, and run the scan again. Running it a second time often succeeds because Windows is in a different state after restart.

What to do if the scan cannot repair files

If you run the scan twice and it still reports that it cannot repair some files, you have a few options. The first is to try running the scan in Safe Mode, which starts Windows with only the essential files and services running. This gives System File Checker a better chance of repairing files that are normally locked by Windows.

To restart in Safe Mode on Windows 10 or 11, click the Start button, then click the power icon in the bottom right corner. Hold down the Shift key on your keyboard and click "Restart" (do not just click Restart without holding Shift). Your computer will restart and show you a menu of startup options. Select "Troubleshoot," then "Advanced options," then "Startup Settings," then click "Restart." When your computer restarts again, press the number 4 or F4 to enter Safe Mode. Once in Safe Mode, open Command Prompt as administrator again and run sfc /scannow.

If Safe Mode does not work, or if your problem persists even after successful repair, the issue may be beyond what System File Checker can fix. At that point, you may want to contact someone with technical knowledge — a friend or family member who knows Windows, a local computer repair shop, or your computer manufacturer's support line.

Preventing file corruption in the future

File corruption usually happens because of a sudden power loss, a forced shutdown, or a failed Windows update. You cannot prevent all corruption, but you can reduce the risk.

Use a surge protector or uninterruptible power supply (UPS) to protect your computer from sudden power loss. Avoid forcing your computer to shut down by holding the power button — instead, use the Start menu and click "Shut down." Let Windows updates install and restart when they ask, rather than postponing them repeatedly. If an update fails, restart your computer and let Windows try again.

If your computer crashes or loses power, do not panic. Run System File Checker the next time you start up, and it will catch most corruption before it causes problems.

Frequently Asked Questions

How long does System File Checker take to run?

The scan usually takes 15 to 60 minutes depending on your drive size and how many files need to be checked. Larger drives and older computers may take longer. Do not interrupt the scan or restart your computer while it is running.

Can I use my computer while System File Checker is scanning?

Yes, you can use your computer normally while the scan runs in the background. Your computer may run slightly slower than usual, but you can open programs, browse the internet, or work on documents without stopping the scan.

What if I see an error message during the scan?

Error messages during the scan are usually not a problem — System File Checker is designed to handle errors and continue scanning. Let the scan finish completely. If an error message appears after the scan finishes, write down the exact text and search for it online, or contact someone who can help troubleshoot.

Do I need to run System File Checker more than once?

If the first scan says it could not repair some files, run it again after restarting your computer. A second run often succeeds because Windows is in a different state. If it still cannot repair files after two runs, try running it in Safe Mode or contact technical support.

Will System File Checker delete my documents or photos?

No. System File Checker only touches Windows system files, not your personal files, documents, photos, or programs. Your files are safe to keep while the scan runs.