Windows Defender Antivirus "Engine Unavailable"

[FireFrost]

I'm running the most recent version of Windows 10. I'm unable to run a scan with Windows Defender AV, nor am I able to update it using Windows Updates. The problem started a few weeks ago - a day after the last successful update of the AV package. I do run third party AV software (Webroot) so there is no immediate threat, but it bugs me that I can't get this fixed.

I've googled extensively and done everything that I could find, including uninstalling Webroot, running sfc, dism, and starting with minimal services going.

Any help resolving this issue would be appreciated.

 

[britechguy]

If you have a third party AV installed, the only thing you should be able to do with Windows Security is to request an offline scan. The installation of a third party solution has, for years, disabled the built-in features of Windows Defender/Security.

When you open Settings, Update & Security, View Update History, scroll down to the Definition Updates section, expand it, and look at when the last definition update was applied and report back. The last one on my system. which is pretty much on 24/7, is KB2267602 with Version 1.351.526.0, that has a date of 10/16/2021.

Please report back on what the last definition update was as a starting point.

 

[britechguy]

I will add, that regardless of the status of the last definition update, if the Windows Security Engine is not starting then my recommendations would be, in order:

1. Using DISM (Deployment Imaging Servicing and Management) and SFC (System File Checker) to Repair Windows 8 & 10

Then, if that does not resolve the problem, next try:

2. Doing a Windows 10 Repair Install or Feature Update Using the Windows 10 ISO file

You have never mentioned the Windows 10 Version or Build that you are currently running. The only difference between a Repair Install or Feature Update is whether the ISO being used is for the same Version/Build, in which case it's a Repair Install, or a later Version/Build, in which case it's a Feature Update. The steps are precisely the same.

BUT REMEMBER, you CANNOT run anything but on-demand offline scans with Windows Defender if you are using a third party security solution. And you'd have no reason to, as they'd be redundant. However, even with a third party solution, Windows Update should still be downloading definition updates because those are used when offline scans are done, so these are kept up to date even with a third-party solution installed.

 

[FireFrost]

Brian,

The last Windows Defender update that installed on my machine was 1.349.1966.0 on 10/5/2021. I started having issues no more than 48 hours later. I may uninstall that update and reinstall and see what happens. This appears to be the second to last update.

I've already ran DISM and SFC. I'm also running Windows 10 version 19043, which I believe is the most current. I'll do a repair install and report back.

 

[britechguy]

I'll do a repair install and report back.

Which is precisely what I'd suggest.

In addition, and related to that Repair Install:

1. Fetch a fresh copy of the ISO if you created one more than two weeks ago.
2. Try doing the repair install with no connection to the internet. It tends to go more quickly, and if there are any updates that weren't in the ISO, Windows Update will apply them afterward.