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10 Tech How-To

Windows Defender Not Working? 7 Fixes to Try

Guide 10-24082 · Updated · Step by step

If Windows Defender (Microsoft Defender Antivirus) is not working on your PC, the fix is usually a combination of restarting its services, checking group policy settings, and ensuring no third-party antivirus is interfering. Below are seven specific fixes that address the most common causes, from disabled services to corrupted system files.

Why Windows Defender Stops Working

Microsoft Defender Antivirus is built into Windows 10 and Windows 11, and it normally runs quietly in the background. However, it can stop working for several reasons:

  • Third-party antivirus conflict: When you install another antivirus program, Windows automatically disables Defender to avoid conflicts. If the third-party app is uninstalled incorrectly, Defender may not re-enable itself.
  • Corrupted system files: A failed Windows update or a disk error can damage Defender’s core files.
  • Group Policy or registry changes: Some software or manual tweaks can disable Defender via policy settings.
  • Stopped services: The Windows Defender service or the Security Center service may not be running.
  • Outdated definitions: If the virus definitions are severely outdated, the app may show an error.

When Defender is not working, you might see a yellow warning icon in the Windows Security app, a message like “This app has been turned off,” or the app might simply fail to open.

Fix 1: Restart the Windows Defender Service

The most common fix is to restart the service that runs Defender. Here’s how:

  1. Press Win + R, type services.msc, and press Enter.
  2. Scroll down to Windows Defender Antivirus Service.
  3. If it is not running, right-click it and select Start.
  4. If it is running, right-click and select Restart.
  5. Also check Security Center service (it may be named “Security Accounts Manager” or “Windows Security Center” depending on your version). Make sure it is running.

If the service fails to start, you may see an error. In that case, try the next fix.

Fix 2: Re-enable Defender via Group Policy or Registry

Windows Pro and Enterprise editions have a Group Policy setting that can disable Defender. If you or a program changed it, you need to set it back.

For Windows Pro/Enterprise:

  1. Press Win + R, type gpedit.msc, and press Enter.
  2. Navigate to Computer Configuration > Administrative Templates > Windows Components > Microsoft Defender Antivirus.
  3. Double-click Turn off Microsoft Defender Antivirus.
  4. Set it to Not Configured or Disabled, then click OK.

For Windows Home: Group Policy is not available, but you can check the registry:

  1. Press Win + R, type regedit, and press Enter.
  2. Go to HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender.
  3. If you see a value named DisableAntiSpyware, right-click it and delete it, or set its value to 0.
  4. Restart your PC.

Fix 3: Check for Third-Party Antivirus Conflicts

If you have another antivirus installed (like Norton, McAfee, or Avast), Windows Defender will automatically turn off. If you uninstalled that antivirus but Defender still won’t start, the uninstall may have left remnants.

To fix this, use the official removal tool from the antivirus vendor (most major vendors provide one). After running it, restart your PC and then check if Defender is active. If not, you may need to manually re-enable it via the Windows Security app:

  1. Open Windows Security (search for it in the Start menu).
  2. Click Virus & threat protection.
  3. Under “Virus & threat protection settings,” click Manage settings.
  4. Turn on Real-time protection if it is off.

Fix 4: Run the Windows Security Troubleshooter

Windows includes a built-in troubleshooter that can automatically find and fix problems with Windows Security.

  1. Open Settings (Win + I).
  2. Go to System > Troubleshoot > Other troubleshooters (Windows 11) or Update & Security > Troubleshoot &strong> (Windows 10).
  3. Find Windows Security (or “System Maintenance” in some versions) and click Run.
  4. Follow the on-screen instructions.

This may take a few minutes. If it finds issues, it will attempt to fix them automatically.

Fix 5: Use PowerShell to Re-register Defender

If the Windows Security app itself is broken, you can re-register it using PowerShell.

  1. Right-click the Start button and select Windows PowerShell (Admin) or Terminal (Admin).
  2. Copy and paste the following command and press Enter:
Get-AppxPackage Microsoft.Windows.SecHealthUI | Foreach {Add-AppxPackage -DisableDevelopmentMode -Register "$($_.InstallLocation)\AppXManifest.xml"}
  1. Wait for it to finish, then restart your PC.

This command re-registers the Windows Security app with the system, which often fixes issues where the app won’t open or shows blank screens.

Fix 6: Run SFC and DISM to Repair System Files

Corrupted system files can break Defender. The System File Checker (SFC) and Deployment Imaging Service and Management Tool (DISM) can repair them.

Run SFC:

  1. Open Command Prompt as administrator (search “cmd”, right-click, and select “Run as administrator”).
  2. Type sfc /scannow and press Enter.
  3. Wait for the scan to complete (this can take 15-30 minutes).

Run DISM:

  1. In the same Command Prompt window, type DISM /Online /Cleanup-Image /RestoreHealth and press Enter.
  2. Wait for it to finish (may take 20-40 minutes).
  3. Restart your PC.

If SFC found and fixed corrupted files, check if Defender works now. If not, you may need to do a repair install of Windows (using the “Reset this PC” option that keeps your files).

Fix 7: Update Windows and Defender Definitions

Sometimes Defender stops working because of a bug that was fixed in a later Windows update. Make sure your system is fully updated.

  1. Open Settings > Windows Update.
  2. Click Check for updates and install any available updates.
  3. After restarting, open Windows Security > Virus & threat protection.
  4. Click Check for updates under “Virus & threat protection updates.”

If you cannot open Windows Security at all, you can download the latest Defender definitions manually from Microsoft’s website (search for “Microsoft Defender Antivirus updates”).

What to Do If Nothing Works

If you’ve tried all seven fixes and Defender still isn’t working, you have a few options:

  • Use a third-party antivirus: While Defender is off, your PC is unprotected. Install a reputable free or paid antivirus to keep your system safe.
  • Perform a repair install: Use the Windows “Reset this PC” feature that keeps your personal files but reinstalls Windows. This often fixes deep system issues.
  • Check for hardware issues: In rare cases, a failing hard drive can cause system file corruption. Run a disk check (chkdsk /f in Command Prompt as admin) to rule this out.

Remember that if you install a third-party antivirus, Windows Defender will automatically turn off. That is normal and not a problem—you don’t need both. But if you uninstall the third-party software later, Defender should turn back on automatically.

Frequently Asked Questions

Why does Windows Defender keep turning off?

The most common reason is a third-party antivirus program. Windows automatically disables Defender when another antivirus is installed to avoid conflicts. If you uninstall that program improperly, Defender may not re-enable itself. Other causes include group policy settings, corrupted system files, or a stopped service.

Can I use Windows Defender and another antivirus at the same time?

No. Windows automatically disables Defender when you install a third-party antivirus. Running two real-time antivirus programs can cause performance issues and conflicts. You should only have one active antivirus. If you uninstall the third-party one, Defender will turn back on automatically.

Is Windows Defender enough protection?

For most home users, yes. Microsoft Defender Antivirus provides real-time protection, cloud-delivered protection, and firewall features. It consistently scores well in independent tests. However, if you are a high-risk user (e.g., you download many files from the internet), you may want a third-party antivirus with additional features like a VPN or password manager.

How do I know if Windows Defender is actually working?

Open the Windows Security app (search for “Windows Security” in the Start menu). If you see a green checkmark under “Virus & threat protection,” Defender is active. You can also check the “Protection history” to see recent scans and detections. If you see a yellow warning or red X, there is a problem.

Will resetting Windows fix Defender?

Yes, a “Reset this PC” that reinstalls Windows will restore all system files and settings to default, which will fix Defender if the issue was caused by corrupted files or policy changes. Choose the option to keep your personal files. Make sure to back up important data before doing this.

What does “Tamper Protection is off” mean?

Tamper Protection is a security feature that prevents other programs from changing Defender’s settings. If it is off, malicious software could disable Defender. You can turn it back on in Windows Security under “Virus & threat protection settings.” If you cannot turn it on, your system may be compromised or a policy is blocking it.

Related

General steps for current mainstream versions; menus can differ by device and version. See the disclaimer or report an error.

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