Windows 11 going sideways — random crashes, apps refusing to launch, an update that won't install, Start menu acting strange? Before you start thinking about a clean reinstall, there are two built-in repair tools that fix a surprising number of these issues: SFC (System File Checker) and DISM (Deployment Image Servicing and Management). At our Edinburgh workshop these are the first things we run when a customer brings in a "weirdly behaving" PC from Stockbridge or Bonnington, and they sort the problem out maybe 60% of the time without touching a single setting.
This guide walks you through the same 5-step sequence we use in the shop. If the commands sound intimidating, don't worry — you're just copying and pasting them. If you'd rather not touch the command line at all, our software troubleshooting team can drive a session remotely from anywhere in Edinburgh and the Lothians.
What SFC and DISM Actually Do
SFC scans every protected Windows system file on your drive, compares each one against a known-good copy stored locally, and replaces anything that's been corrupted, modified or deleted. Think of it as a spell-check for Windows itself.
DISM works one layer deeper — it repairs the local store of "known-good" files that SFC relies on. If that store is itself damaged (which is common after a botched update), SFC has nothing clean to copy from, and DISM has to fix the source material first by pulling fresh files from Windows Update.
Used together, in the right order, they cover the vast majority of "Windows is acting weird" complaints we see from Murrayfield to Linlithgow.
When You Should Run These Commands
The classic symptoms that SFC/DISM tend to fix:
- Windows Update keeps failing with cryptic error codes
- Random app crashes, especially built-in apps like Settings, Photos or Calculator
- Start menu, search bar or taskbar misbehaving
- Blue screens that mention system_service_exception or critical_process_died
- Strange UI glitches that survive a reboot
- Errors after a power cut or hard shutdown
If your symptoms are more dramatic — a PC that won't boot at all, or a flickering blue screen on startup — our guide to fixing the blue screen of death is a better starting point. And before you run any repair, back up your important files. Hardware faults can look like software issues, and if a failing drive is the real culprit, you don't want to lose data trying to fix it. Our data recovery service handles those cases.
1. Open Command Prompt as Administrator
Both tools need administrator rights. To open an elevated Command Prompt:
- Click the Start menu (or press the Windows key).
- Type cmd.
- Right-click Command Prompt in the results and choose Run as administrator.
- Approve the User Account Control prompt.
You should see a black window with a title bar that reads Administrator: Command Prompt. If it doesn't say "Administrator" at the top, close it and try again — without admin rights the commands will refuse to run.
Windows Terminal works just as well — right-click the Start button and pick Terminal (Admin). Either gets you to the same place.
2. Run an SFC Scan
In the elevated prompt, type the following and press Enter:
sfc /scannow
The scan walks through every protected system file. Expect it to take 10–30 minutes depending on your drive speed; an NVMe SSD finishes in a few minutes, an older SATA drive can take half an hour. Don't close the window.
When it finishes, you'll get one of four messages:
- Did not find any integrity violations — Windows files are clean. Move on to DISM only if symptoms persist.
- Found corrupt files and successfully repaired them — Done. Reboot and test.
- Found corrupt files but was unable to fix some of them — You need DISM. Continue to step 3.
- Could not perform the requested operation — Almost always means the component store is damaged. Run DISM.
3. Run DISM to Repair the Component Store
If SFC couldn't finish the job, DISM is the next stop. Type:
DISM /Online /Cleanup-Image /RestoreHealth
This command checks your local component store for corruption and downloads fresh replacement files from Windows Update. You need a working internet connection for it. If your Wi-Fi is unreliable, plug in an Ethernet cable for this step — a dropped connection halfway through will leave the repair incomplete.
DISM is slower than SFC. The progress bar often appears to stall at 20% for several minutes, then jumps. That's normal — don't cancel it. Total time is usually 15–45 minutes. We've had a few stubborn machines from Falkirk and Glasgow take well over an hour, so leave it running and grab a coffee.
Look for the message The restore operation completed successfully. If you get an error, note the code — it's almost always a Windows Update issue, and our Windows Update errors guide covers the usual suspects.
4. Run SFC One More Time
Now that DISM has refreshed the component store, SFC has a clean source to repair from. Run it again:
sfc /scannow
This time, files that SFC couldn't fix on the first pass should be repaired. If you still see "unable to fix some of them" after this second run, the corruption goes deeper than these tools can reach — see step 5.
5. Reboot and Verify the Fix
Restart the PC. SFC and DISM make most changes immediately but a few only fully take effect after a clean reboot. After restart, retry whatever was failing originally — install the stuck update, open the misbehaving app, look for the missing Start menu items. If everything works, you're done.
Worth knowing: SFC writes a detailed log at C:\Windows\Logs\CBS\CBS.log. If you want to see exactly which files it repaired, that's where to look — though it's a long, dense file, and not particularly fun reading.
Common Errors and What They Mean
A few of the messages we see most often:
- "Windows Resource Protection could not start the repair service" — the Windows Modules Installer service is disabled. Open services.msc, find it, set Startup type to Manual, and try again.
- DISM error 0x800f081f — the source files are missing or corrupted. Sometimes a fresh Windows ISO from Microsoft is needed as a repair source.
- DISM error 0x800f0906 — Windows Update can't be reached. Check your network and any VPN that might be blocking Microsoft endpoints.
- "You must be an administrator running a console session" — your Command Prompt isn't elevated. Close it and reopen with Run as administrator.
When SFC and DISM Aren't Enough
If both tools finish cleanly but your PC still misbehaves, the issue isn't corrupted system files — it's something else. The usual next steps are an in-place upgrade (running the Windows 11 installer over the top of itself, keeping files and apps), then a full clean install if that doesn't help. Our guide on how to clean install Windows 11 covers that route, and our OS installation team can do the whole thing for you — including a full data backup, driver setup, and app reinstall — at our Edinburgh workshop.
If you'd rather not touch any of this yourself, our remote support service lets us run SFC, DISM and the follow-up diagnostics over a secure remote session, with no need to bring the machine in.
Need a Hand?
SFC and DISM are powerful, but they're not a cure-all. If you've tried both and Windows 11 is still acting up, bring the machine to our Parkhead Drive workshop or book a remote session. We cover Edinburgh, the Lothians and beyond — and we'll tell you straight whether it's a software fix or something deeper. Book a repair online in under two minutes.