Signature update issues can’t always be resolved by treating them as Windows Update issues per se, and can often occur when Windows Update itself is otherwise working as expected. So any troubleshooting guide for definition update issues should include
steps for resolving any underling issues with the operating system and any operational issues with Windows Defender itself, since Defender’s definition updates often seem to fail due to a problem in the way that Defender is calling, installing, or processing
its signature updates.
Before proceeding with the advanced troubleshooter, first restart the PC; and then visit the manual signature update download page. Follow the advice there to trigger a manual update via the Windows Defender Security Center app, since:
A manually triggered update immediately downloads and applies the latest definitions. This process might also address problems with automatic updates.
If the WDSC manual update isn’t successful, then download the definition update and attempt to install it, since this also might resolve the issue.
Since operational issues with Windows Defender itself are commonly caused by undetected malware, unremoved antimalware components, or system corruption – the standard remedies for these issues should probably be applied prior to running the Window
Update Troubleshooter, not only because these sources would be more suspect in a case where there wasn’t any general issue with Windows Update – but also because these factors would likely prevent the Windows Update Troubleshooter from succeeding, and because
there might actually be an issue with the Troubleshooter itself if the system is in a corrupted state. Also note that checking for missed malware and system corruption should be considered routine maintenance chores for most users:
So here’s what I would recommend for persistent signature update issues:
First run the removal tools for any previously installed or preinstalled AV apps:
And then run the standard Windows 10 system repair tools to reverse any system corruption or damage. This step is absolutely essential, and should be performed prior to running the Windows Update
More Importantly: the Windows Defender Command Line Utility has its own built-in tool for resolving signature update issues by purging the signature cache, and you can run this tool if the signature
update command fails:
Type “cmd” in the search box, then right-click on “Command Prompt” and select “Run as administrator”; and then copy, paste, and enter this command at the prompt:
Keep in mind that Windows Defender is a replaceable component of Windows 10, and that the simplest solution for intractable Windows Defender signature update issues might be to just install a free third-party AV app.