If Microsoft Defender scan is not working, the problem is usually tied to disabled services, corrupted security components, blocked updates, or conflicting antivirus software.
This guide explains how to fix Microsoft Defender scan not working with practical steps that restore Windows Security scanning on Windows 11 and Windows 10.
Why Microsoft Defender scans stop working
Microsoft Defender Antivirus is built into Windows Security and depends on several system components working together.
When one piece fails, quick scans, full scans, or scheduled scans may stop, freeze, or return errors.
Common causes include:
- Real-time protection or tamper protection being disabled
- The Microsoft Defender Antivirus Service not running
- Corrupted Windows Security app data
- Outdated virus definitions or Windows updates
- Third-party antivirus software taking over scanning
- System file corruption affecting security components
Check Windows Security and Defender status
Start with the Windows Security interface because it often reveals the root problem immediately.
Open Windows Security, then go to Virus & threat protection and look for warnings, expired definitions, or messages that scanning is unavailable.
If you see “Your virus and threat protection is managed by your organization,” group policy or device management may be controlling Defender.
On personal devices, that message can also appear after a registry change, a third-party antivirus install, or a policy leftover from previous software.
Restart the required Defender services
Defender scanning depends on background services that must be running continuously.
If they stop, scans may fail to start or end immediately.
- Press Win + R, type services.msc, and press Enter.
- Find Microsoft Defender Antivirus Service and Windows Security Service.
- Confirm their startup type is not disabled.
- If a service is stopped, right-click it and select Start.
- Restart the computer and try a scan again.
If the services refuse to start, that usually points to policy restrictions, damaged system files, or another antivirus product interfering with Windows Defender.
Verify that real-time protection and tamper protection are enabled
Microsoft Defender may stop scanning properly when real-time protection or tamper protection is turned off.
These settings help prevent malware and unauthorized changes from disabling antivirus features.
Open Windows Security, select Virus & threat protection, then Manage settings.
Make sure Real-time protection is on.
If available, also turn on Tamper Protection.
On managed devices, tamper protection may be controlled by an administrator.
If you cannot change it, that does not always mean Defender is broken, but it can limit what troubleshooting steps you can use.
Update Microsoft Defender definitions and Windows
Outdated security intelligence can cause scan failures, false errors, or incomplete scan results.
Defender uses regular definition updates to detect current threats and maintain compatibility with Windows components.
To update manually:
- Open Windows Security
- Go to Virus & threat protection
- Select Protection updates
- Click Check for updates
You should also install pending Windows Update patches.
Cumulative updates often fix Defender engine issues, especially after major feature updates in Windows 11 or Windows 10.
Remove third-party antivirus software
Another antivirus program can disable Microsoft Defender scanning or partially take control of security functions.
Even after uninstalling the software, its drivers or leftover policy settings can continue to interfere.
Check whether you have security software from vendors such as Norton, McAfee, Bitdefender, Kaspersky, ESET, Avast, or Trend Micro.
If so, uninstall it completely and restart your PC.
Then confirm that Microsoft Defender becomes the active antivirus again.
If Defender works only after removal, the issue was likely an antivirus conflict rather than a broken Windows component.
Run the scan from PowerShell
If the Windows Security interface is unresponsive, start a scan using PowerShell.
This helps determine whether the problem is limited to the app or affects Defender itself.
Open PowerShell as administrator, then use the Defender cmdlet for a quick scan command supported on many Windows builds.
If the command works but the GUI does not, the issue may be with the Windows Security app rather than the antivirus engine.
PowerShell is also useful for checking Defender status, update state, and protection history without relying on the standard interface.
Repair system files with SFC and DISM
Corrupted Windows files can break Defender scanning because the antivirus engine depends on core operating system components.
Running built-in repair tools can restore those files without reinstalling Windows.
- Open Command Prompt as administrator.
- Run sfc /scannow and wait for completion.
- If problems remain, run DISM /Online /Cleanup-Image /RestoreHealth.
- Restart the PC after the repairs finish.
Use SFC first because it checks local protected files.
Use DISM next if Windows image corruption prevents SFC from repairing everything.
Reset the Windows Security app
The Windows Security app can itself become damaged, which may prevent Defender scans from launching correctly.
Resetting the app often clears cached errors and restores normal behavior.
Go to Settings > Apps > Installed apps, find Windows Security, then open Advanced options.
Choose Repair first.
If that does not help, use Reset.
After the reset, reopen Windows Security and test both a quick scan and a full scan.
Check Group Policy and registry settings
In some cases, Defender is disabled by policy settings that were applied by software, scripts, or previous administration tools.
This is especially relevant if the device was once managed by an organization.
Look for policies that disable Microsoft Defender Antivirus, turn off real-time protection, or prevent scan actions.
On unsupported devices, registry keys may also block Defender services.
Be careful when editing policy or registry settings, because incorrect changes can weaken security or cause additional problems.
Try a clean boot if scans still fail
A clean boot helps identify whether a background app or service is blocking Defender.
This is useful when scans fail without clear error messages.
- Open System Configuration by typing msconfig in the Run dialog.
- Hide Microsoft services, then disable the remaining non-Microsoft services.
- Disable startup apps in Task Manager.
- Restart the computer and test Microsoft Defender scanning.
If scanning works in a clean boot state, re-enable items gradually until you identify the conflict.
When to use offline scanning
Some malware actively blocks antivirus actions while Windows is running.
In those cases, normal scans may not start, or they may close immediately.
Use Microsoft Defender Offline scan from Windows Security when available.
This mode restarts the computer and scans before Windows loads fully, which can bypass active threats that interfere with Defender.
Offline scanning is especially helpful if scans fail after signs of infection such as browser redirects, changed homepage settings, unknown startup items, or repeated security alerts.
What to do if Microsoft Defender still will not scan
If you have already checked services, updates, repairs, app reset, and antivirus conflicts, the issue may be deeper in the Windows installation.
At that point, consider creating a new local user profile to rule out profile corruption, or perform an in-place repair upgrade to repair Windows while keeping files and apps.
For IT-managed devices, contact the administrator because policy controls may intentionally limit Defender scanning.
For personal PCs, if malware is suspected and offline scans fail, backup important files and seek professional support before making major changes.
Quick checklist for fixing scan failures
- Open Windows Security and review alerts
- Restart Microsoft Defender and Windows Security services
- Enable real-time protection and tamper protection
- Update Defender definitions and Windows
- Remove third-party antivirus software
- Run SFC and DISM repairs
- Reset the Windows Security app
- Test a clean boot and offline scan