What a system file check does and when to run one

A system file check is a built-in tool that scans your operating system files for corruption or damage and repairs them automatically. Windows calls this tool SFC (System File Checker); Mac calls it Disk Utility or First Aid. You run it when your computer is crashing unexpectedly, freezing during normal tasks, or showing error messages about missing or damaged files — problems that a restart alone won't fix.

The scan takes 15 minutes to an hour depending on your drive size and how many files need checking. It requires administrator access and works best when you run it from a command line rather than through the graphical interface, because the command-line version can actually repair files while Windows is running. On Mac, you typically run it from Recovery Mode, which boots your computer into a special maintenance environment.

System file checks are safe to run repeatedly. They don't delete your documents, photos, or programs — they only touch the operating system files that Windows or macOS needs to function. If the tool finds corruption, it attempts to restore the correct version from a backup copy stored on your drive.

Key Takeaways

  • Windows users open Command Prompt as administrator and type sfc /scannow to scan and repair system files in one step.
  • The scan takes 15 minutes to an hour and requires your computer to stay plugged in and undisturbed until it finishes.
  • Mac users restart their computer while holding Command + R to enter Recovery Mode, then select Disk Utility and run First Aid on the main drive.
  • If the Windows scan finds corruption but cannot repair it, you may need to run DISM (Deployment Image Servicing and Management) as a follow-up step.
  • A successful scan does not may provide your problem is solved — file corruption is one possible cause among many, so document what happens after the scan completes.

Running a system file check on Windows 10 and 11

Open the Start menu and type Command Prompt. Right-click the result and select "Run as administrator" — this is the critical step, because the tool cannot repair files without administrator permission. If you see a dialog asking "Do you want to allow this app to make changes to your device?", click Yes.

In the Command Prompt window, type sfc /scannow (with a space between sfc and the forward slash) and press Enter. The scan will begin when ready. You will see a progress bar and a message saying "Scanning" followed by a percentage. Do not close this window, restart your computer, or unplug it while the scan is running — interrupting it can leave your system in a worse state.

When the scan finishes, you will see one of three messages. "No integrity violations were found" means your system files are intact and this tool is not the source of your problem. "The Windows Resource Protection found corrupt files and successfully repaired them" means the tool fixed the issue — restart your computer and test whether your original problem is resolved. "The Windows Resource Protection found corrupt files but was unable to fix some of them" means you need to run a second tool called DISM before trying SFC again.

Running DISM if SFC cannot repair files

If SFC reports that it found corrupt files but could not repair them, the issue may be in the system image that SFC uses as a reference. DISM (Deployment Image Servicing and Management) can rebuild that image. Open Command Prompt as administrator again and type DISM /Online /Cleanup-Image /RestoreHealth (note the spaces and forward slashes exactly as shown).

This scan takes longer than SFC — often 20 to 45 minutes — and requires an internet connection because it may need to read replacement files from Microsoft. Let it run to completion without interruption. When it finishes, run SFC again using the same sfc /scannow command. This time, SFC should be able to repair the files it found.

If DISM or SFC report errors during the process, write down the exact error code and message. These details help a technician diagnose whether the problem is hardware-related (a failing drive) or software-related (a corrupted Windows installation that may need repair or reinstall).

Running a system file check on Mac

Shut down your Mac completely. Press the power button to turn it on, then when ready hold down Command + R and keep holding until you see the Apple logo or a spinning globe. This boots your Mac into Recovery Mode, a separate environment where you can repair the main drive without it being in active use.

When Recovery Mode finishes loading, you will see a menu with several options. Click "Disk Utility" and then click "Continue". In the window that opens, select your main drive (usually named "Macintosh HD" or similar) in the left sidebar, then click the "First Aid" button at the top. Click "Run" when prompted.

First Aid will scan your drive and report any problems it finds. If it finds issues, it will attempt to repair them automatically. When the scan completes, you will see a summary message. If it says your drive is okay, restart your Mac normally. If it found and repaired issues, restart and monitor whether your original problem persists. If First Aid cannot repair the problems it finds, your drive may have hardware damage and you should back up your files and contact Apple Support or a repair technician.

What to do after the scan completes

Restart your computer normally and use it as you would for the next few hours. Pay attention to whether the problem that prompted you to run the scan still occurs. If you were experiencing crashes, freezing, or error messages, note whether they continue, happen less often, or stop entirely.

If the problem persists after a successful scan, the issue is likely not file corruption. Common alternatives include a failing hard drive (which shows up as slow performance or clicking sounds), insufficient RAM for the programs you are running, malware or a virus, or a hardware driver that needs updating. Document what you observed — the exact error message, what program was running when it happened, how often it occurs — because this information narrows down the next step.

If the scan found corruption and repaired it, but the problem returns weeks later, it may indicate a failing drive that is gradually developing bad sectors. In that case, back up your important files and consider replacing the drive before it fails completely.

When to seek help from a technician

If you cannot open Command Prompt as administrator on Windows, or if your Mac will not boot into Recovery Mode, the problem may be more serious than file corruption. A technician can run these tools from external media or perform a deeper diagnosis of your hardware.

If the scan completes but reports errors you do not understand, or if DISM reports that it cannot read replacement files, write down the exact error code. This code is specific enough that a technician can look it up and determine whether your drive is failing, whether your Windows installation is severely damaged, or whether you need a different repair approach.

If your computer will not start at all, or starts but when ready crashes before you can open Command Prompt, you may need to use a bootable USB drive or external drive to run these tools. This is beyond what most users can do alone, and a technician can set this up for you.

Frequently Asked Questions

Can I use my computer while the system file check is running?

No. On Windows, close all other programs and do not use your keyboard or mouse while SFC is scanning. On Mac, your computer is in Recovery Mode and you cannot use it normally. Both scans require the operating system to be mostly idle so they can access system files without interference.

What if the scan says it found corrupt files but I do not notice any problems?

File corruption does not always cause when ready symptoms. The corrupted files might be ones your computer rarely uses, or they might cause problems only under specific conditions. Running the repair is still worthwhile because it prevents those files from causing crashes later. Monitor your computer over the next week to see if performance improves.

Do I need to run this scan regularly as maintenance?

No. Run a system file check only when you are experiencing actual problems like crashes or error messages. Running it as routine maintenance does not prevent problems and is unnecessary if your computer is working normally. A healthy system will report no integrity violations.

What is the difference between SFC and DISM?

SFC checks system files against a reference image stored on your drive. DISM rebuilds that reference image using files from Microsoft. If SFC cannot repair files, it is usually because the reference image itself is corrupted, which is why you run DISM first to fix the reference, then run SFC again.

Will this scan find and remove viruses?

No. A system file check only repairs operating system files. It does not scan for malware or viruses. If you suspect a virus, run a dedicated antivirus or antimalware scan separately using Windows Defender (built into Windows) or a third-party tool like Malwarebytes.