SFC /scannow, DISM and CHKDSK: how to check and repair Windows, step by step

If we sent you this link, these are the steps we would otherwise type out for you in an email, in the order we run them. They are Windows’ own repair tools: you type a short command into a window, and Windows checks itself and fixes what it can. We’ve seen these checks put a misbehaving computer right more than once. If they can’t fix it, book home tech support if these checks don’t fix it.

Take it one step at a time. Each step tells you what to type, roughly what to expect, and what to do when the message comes back.

Is it the computer or the internet?

Check that first, because these commands repair Windows, not your internet connection. Plenty of people blame the internet when the problem is the computer, and the other way round. If websites are slow on every device in the house, it is the connection. If only this computer is slow, even with programs that don’t use the internet, it is the computer.

Is it my computer or my internet? sorts that out in two minutes, with tests anyone can do. If it is the computer and it is a Mac, the Mac checks with Disk Utility First Aid are the ones you want. If it is a Windows computer, read on.

What do SFC, DISM and CHKDSK actually do?

They are three checks built into Windows, each for a different part of the computer. DISM repairs the store of files Windows uses to mend itself. SFC, the System File Checker, repairs Windows’ own system files from that store. CHKDSK checks the drive the files are kept on.

Microsoft’s own page says to run DISM before SFC, so that is the order here: DISM, then SFC, then CHKDSK. None of them is a virus scan, and none of them speeds up a computer that is simply old or full. They repair damage to Windows and to the drive’s records of where things are kept, and that is all.

Back up your photos and documents first

Back up before you start. These checks repair Windows itself, but a computer that needs them can also have a drive that is starting to fail, and a backup is the only thing that protects your photos if it does. Windows 11 and Windows 10 both have a built-in backup called File History, which copies your libraries, such as Documents, Pictures and Videos, to an external drive.

You need an external drive, the kind that plugs into a USB port, with more free space than the files you are copying.

We earn a small commission if you buy through our Amazon links, at no extra cost to you. As an Amazon Associate we earn from qualifying purchases.

1. 1

   Plug in the external drive

   Plug it into a USB port on the computer and wait a moment for Windows to find it.

2. 2

   Open Control Panel

   Select Search on the taskbar, type control panel, and open it. Under System and Security, select Save backup copies of your files with File History.

3. 3

   Turn it on

   Select Turn on. If no drive is chosen for you, select Select drive, pick the external drive, and select OK.

4. 4

   Let it copy

   Leave the drive plugged in while File History copies your files, then you can start the checks.

Microsoft’s guide to File History (opens in a new tab) has the restore steps too, for the day you need a file back. If your photos are already backed up somewhere else, such as a cloud account you know works, you can skip this part.

Photos still on your phone? Here’s how to copy phone photos to the PC and back them up.

While you’re in Windows, check Microsoft Defender is switched on too.

If Windows ever starts on a blue screen asking for a recovery key instead, where to find your BitLocker recovery key has the steps.

On Windows 10? Microsoft no longer fixes it; your options now Windows 10 support has ended are on our guide.

How do I open Command Prompt as administrator?

Search for command prompt, then choose Run as administrator. Every command on this page needs the window opened this way, and the steps are the same on Windows 11 and Windows 10:

1. 1

   Search for it

   Select Search on the taskbar, at the bottom of the screen, and type command prompt.

2. 2

   Choose Run as administrator

   When Command Prompt appears in the results, select Run as administrator, which sits to the right of it. You can also right-click Command Prompt and choose Run as administrator.

3. 3

   Say yes

   Windows asks for permission. Select Yes, or type the administrator password if it asks for one.

4. 4

   A black window opens

   That is Command Prompt. It is waiting for you to type. Nothing happens until you type a command and press Enter.

On Windows 11 there is also a shortcut: press the Windows logo key and X together, then choose Windows Terminal (Admin); on your screen it may just say Terminal (Admin). Select Yes. The commands below work in that window too.

For each command below, copy the command, or type it exactly as shown, then press Enter. To paste a command you have copied, right-click once inside the black window. If nothing appears, type it instead, exactly as shown, spaces and slashes included.

Step 1: repair Windows’ repair store with DISM

DISM, the Deployment Image Servicing and Management tool, checks and repairs the store of files Windows uses to mend itself. It uses Windows Update as the source of the files it needs, so the computer must be connected to the internet while it runs.

DISM.exe /Online /Cleanup-image /Restorehealth

Leave it alone until it finishes and the window is waiting for you to type again. Read the last lines it printed:

• “The restore operation completed successfully” means the repair store is fixed or was fine already. Go on to Step 2.
• Anything else, usually an error with a code, means DISM could not finish the repair. Write the code down, then go on to Step 2 anyway. If Step 2 also cannot fix things, the code is useful to whoever helps you next.

Want to look before you repair?

DISM has two checks that look without repairing. You don’t need them, because the repair above does the looking as well, but some people like to see where they stand first.

Dism /Online /Cleanup-Image /CheckHealth

Dism /Online /Cleanup-Image /ScanHealth

If CheckHealth says repairable, run the repair command at the top of this step.

Step 2: run sfc /scannow

The System File Checker, sfc /scannow, scans every protected Windows system file and repairs the ones with problems where it can. It is the check most people have heard of, and Microsoft runs DISM before it so that it has good copies to repair from.

sfc /scannow

Microsoft’s instruction is plain: do not close the Command Prompt window until the verification is 100% complete. Leave it running, go and make a cup of tea, and come back to it.

When it finishes, it prints one of four messages. These are Microsoft’s words, so you can match yours exactly:

Microsoft’s next step after “unable to fix some of them” is finding the damaged files in a log and replacing them by hand. That is a job for a technician, not something to try from a web page. The full detail is on Microsoft’s System File Checker page (opens in a new tab) if whoever helps you wants it.

Step 3: check the drive with CHKDSK

CHKDSK, Check Disk, looks for errors in the file system on the drive, the records Windows keeps of where every file sits. Run the scan first. It looks for problems without making any changes.

chkdsk /scan

When it finishes, read the last few lines. If it reports nothing that needs fixing, the drive’s records are fine and you are done with CHKDSK. If it reports errors that need fixing, run the fix below. If you can’t tell which, take a photo of the screen before you go on.

Fixing errors at the next restart

The fix needs the drive to itself, and Windows is running from it, so CHKDSK offers to do it the next time the computer starts.

chkdsk /f

Windows will answer with this question, word for word:

Type Y and press Enter. That only books the check; nothing happens until you restart. Save anything you have open, then restart the computer. As it starts up, CHKDSK checks the drive and corrects the errors it finds, then restarts the computer again by itself.

On a large drive this can take a long time. Leave it to finish: Microsoft says interrupting CHKDSK isn’t recommended, and you can’t open your files until it is done. If you typed N by mistake, run the command again and type Y.

CHKDSK has other switches. Leave them alone: the two above are all a home computer needs.

Freezing or crashing? Run Windows Memory Diagnostic

Windows Memory Diagnostic tests the computer’s memory, the part that holds whatever it is working on, and faulty memory can cause freezes and crashes. It is opened from Control Panel, not typed as a command.

1. 1

   Save your work

   Save anything you have open and close your programs first.

2. 2

   Open Control Panel

   Select Search on the taskbar, type control panel, and open it.

3. 3

   Find the memory check

   In the search box at the top right of Control Panel, type memory. Select Diagnose your computer's memory problems.

4. 4

   Follow the window

   Do what the window that opens asks. Leave the computer alone until the test has finished.

Microsoft says the results are kept in Event Viewer, in the System log, as an entry called MemoryDiagnostics-Results (Microsoft’s troubleshooting tips (opens in a new tab), checked 20 October 2026). That is fiddly to find, so if you’d rather not, just tell whoever helps you that you ran it. A memory fault is a hardware fault, which is a job for a repair shop.

When it’s the connection, not the computer

If this computer is the only one that can’t get online, or websites won’t load while other programs work, Windows’ network settings may be the problem. Microsoft gives five commands to reset them. Run them in this order, in Command Prompt opened as administrator, one at a time, pressing Enter after each.

netsh winsock reset

netsh int ip reset

ipconfig /release

ipconfig /renew

ipconfig /flushdns

That last one is the flush DNS people search for. Windows keeps a list of website addresses it has looked up, called the DNS cache, and this command clears it. After the five commands, check whether your connection works.

Network reset, the bigger step

If the commands don’t fix it, Windows has a Network reset in Settings, the last step Microsoft suggests. It removes the network adapters and their settings, then puts them back with default settings when the computer restarts.

1. 1

   Open Network & internet

   Select the Start button, type settings, open Settings, then select Network & internet.

2. 2

   Find Network reset

   On Windows 11, select Advanced network settings, then Network reset. On Windows 10, select Status, then Network reset.

3. 3

   Reset now

   Select Reset now, then Yes. Save your work first, because the computer restarts.

4. 4

   Reconnect

   After the restart, join your WiFi again. Keep the WiFi password from your modem's label handy in case it asks.

Microsoft notes that a VPN or other networking software may need setting up again afterwards. The steps are on Microsoft’s page for fixing WiFi connections (opens in a new tab).

If every device in the house has the same trouble, none of this will help, because the problem is the modem, the WiFi or the line. Start with restarting the modem and NBN box in the right order, then what “connected, no internet” means. To see whether the connection itself is steady, ping, latency and packet loss, shown explains the test and what a good result looks like. A connection that drops for the whole house is a job for an internet technician who tests the line and the modem, and WiFi that won’t reach the far rooms is one for a WiFi technician.

When to stop and call us

Stop when a check says it cannot repair something, or when the same problem comes back after every check is clean. You don’t need us if DISM, SFC and CHKDSK all came back clean or fixed what they found, and the computer now behaves. That is the whole job done, and it is yours.

Call us when:

• sfc /scannow says it was unable to fix some files twice, after DISM.
• sfc /scannow says it could not perform the requested operation, or DISM fails with the same error code twice.
• CHKDSK keeps finding errors each time you run it, which can mean the drive is failing.
• The same freeze, crash or blue screen keeps coming back after every check is clean, or the memory test finds a fault.
• You’d rather not do it yourself. That is a fair call, and these checks are part of what our home tech support visit works through.

Be clear on what a visit covers. We do setup and help: we run these checks with you, sort out Windows and the connection, and show you what we did. We don’t repair hardware, so no screens, parts or batteries, and we don’t do data recovery. If the drive is failing, we tell you straight away, and the visit goes on what can be sorted, starting with getting your files backed up while the drive still reads.

Not sure whether it is worth a visit? Send us a message with the exact message Windows gave you, or ring 02 9188 1577.