FIX: Windows Defender protection definition update failed

Aleksandar Ognjanovic
by Aleksandar Ognjanovic
Troubleshooting Expert
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  • This can be a pretty common problem and is most often encountered when your Windows updates failed. That's why we made sure to give you the right steps in this article for this particular situation.
  • Windows 10 users are depending on their PCs on a daily basis for lots of productivity tasks, but everything can go wrong if your Windows antivirus is not updated.
  • Windows Defender protection definition update failed and other similar errors can be easily fixed if you follow the right steps. Check out our complete System Errors hub for more info.
  • Don't forget to bookmark our dedicated Windows 10 errors hub for more articles like this.
how to fix Windows Defender protection definition update failed

Windows Defender is slowly but steadily getting a lot more trust from users. On the other hand, a lot of errors from the current and previous Windows 10 major releases, are still an issue.
One common issue comes with a variation of the error codes and accompanied by the Protection definition update failed message.

Today we’ll try to help you address this. In case you’re stuck with the error at hand, make sure to check the steps we provided below.

  1. Delete temporary files
  2. Restart Windows Defender service
  3. Update Defender manually
  4. Run SFC
  5. Reset your PC

How do I fix protection definition update failed?

1. Delete temporary files

  1. In the Windows Search bar, type Disk and open Disk Cleanup tool.protection definition update failed
  2. Choose the System partition (C:) and click OK.protection definition update failed
  3. Click on the Cleanup system files.protection definition update failed
  4. Check the Temporary files box and click OK to clean them completely.protection definition update failed
  5. Now, boot into a Safe mode and navigate to C:\Windows\Software Distribution and delete the content in the folder.
  6. Restart your PC and look for changes in Defender behavior.

The corruption within the system files, especially the ones concerning update distribution and storage is a rather common occurrence. In this scenario, the previous iteration of the Windows Defender virus definition might be corrupted. In order to address this, we recommend clearing all temporary files and restarting the update procedure.

Windows Defender usually updates its definition automatically on a frequent basis and stores it in a dedicated folder. Deleting the content of the folder might help, as well.

After this, just restart your PC and make sure that it is connected to the internet. Windows Defender should do the rest.

2. Restart Windows Defender service

On the other hand, if the Defender itself shows signs of stalling or misbehaves in an unusual manner, there are additional steps to take. Even though Windows Defender is the built-in part of the Windows 10 shell, it can be disabled. It automatically turns off in presence of a third-party antivirus. Now, even if you uninstalled the third-party antivirus prior to the occurrence of the update error, we encourage you to check the dedicated service.

In order to do so, follow these steps:

  1. Press Windows + R to summon the Run command line.
  2. Type services.msc and press Enter.protection definition update failed
  3. Navigate to the Windows Defender individual services.protection definition update failed
  4. Right-click on it and open Properties on every service respectively.
  5. Make sure every single one is set to start Automatically.

3. Update Defender manually

  1. Navigate to this site and download the latest update.
  2. Install updates and restart your PC.
  3. Try running Windows Defender again.

Furthermore, updates aren’t Windows Update exclusives. Namely, you can download the recent definition for Windows 10 manually and install it just like any other program. We all know how Windows Update for Windows 10 frequently causes issues of different kinds. That makes this nothing new.

Updating Windows Defender’s definition is rather simple.

4. Run SFC

  1. In the Windows Search bar, type CMD, right-click on the Command Prompt and run it as an admin.protection definition update failed
  2. In the command line, type sfc/scannow and press Enter.protection definition update failed
  3. When everything is done, close the elevated command line and restart your PC.

If the built-in, essential part of Windows 10 won’t work as intended, you can’t reinstall it. What you can do is run the System File Checker and hope for the best. This built-in system utility runs through the elevated Command Prompt. Its main purpose is to check for the corruption within the system files. Once it gets what’s wrong, SFC will try to repair the affected files.

5. Reset your PC

Finally, if you’re still unable to resolve the issue at hand, we can go as far as recommending to reset your PC to factory settings. It might seem discouraging at the first glimpse, but your files and apps won’t get affected whatsoever. In addition, the procedure is simple and quick – much better than the last resort. Which is, of course, a clean system reinstallation. After you reset your PC to factory values, the Windows Defender update error should be gone.

Here’s how to reset your system to factory settings in a few simple steps:

  1. Firstly, backup your data.
  2. Once you’ve done that, press Windows key  + I to open the Settings app.
  3. Open the Update & security section.protection definition update failed
  4. Select Recovery in the left pane.
  5. Click on the Get Started button under the Reset this PC section.protection definition update failed
  6. Choose to Keep your files and click Next.protection definition update failed

That’s it. We hope at least one of the aforementioned solutions was helpful in addressing the Protection Definition update failed error in Windows 10. In case you’re aware of some alternatives or have a question regarding the ones we enlisted, make sure to tell us in the comments section below.

FAQ: Learn more about this issue

  • How to update windows 10 defender virus definitions?

From the Settings app go to go to Update & security menu and then open Windows Update. Select Check for updates, Windows 10 will also download and install definitions for Defender.

  • Does Windows Defender update automatically?

If you want windows defender to be updated automatically, then select the option Automatic or Install updates automatically. However,  all updates of windows will be installed automatically like windows defender.

Editor’s Note: This post was originally published in May 2018 and it has been since revamped and updated in April 2020 for freshness, accuracy, and comprehensiveness.

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