Q: Windows Defender Refuses to Turn ON :(

I woke up this morning (September 30, 2016) when someone from "Windows Tech Support" called me saying that my PC was sending them some "mysterious errors"! (I know... They were those darn scammers....)

However, to my surprise when I booted my PC to start my work, a message from Windows Defender appeared saying that I needed to "scan my PC", so I did, but Oh Surprise!!!

I tried to update the virus definition files and it failed, then the problem started!

As many have reported here, WD refuses to Turn ON!! :( WHY?

It was working perfectly ever since I upgraded to Windows 10! I do have other antivirus installed, but they are not actively scanning my machine, only on demand because I thought that WD would do its job.

Here is a screenshot of my issue:

As mentioned before, it was working fine as I scanned my PC two days ago with no issues.

What has happened? What changed? Is there a new update that breaks WD? What is error code 0x800705b4? Why WD cannot be enabled?

I see endless posts here and I'm not sure if they actually solve the problem. They just tell you go here, here, there, over there, read more stuff, etc and nothing seems to work.... :(



Great questions! But IMO there are certainly no clear answers... 

I'm sure you restarted and again tried a manual update.

Here is some reading material you may find of interest:  Note the "block" near the bottom of this document (from Microsoft) that specifically states Windows Defender automatically turns off if you install another antivirus app.

Sounds like something may have gone wrong if you’re running more than one antivirus program for antivirus protection.

Using more than one antivirus program that provides real time scanning may cause software conflicts and performance issues.  Not long after I first booted up my new Dell Inspiron (w/ Win 10 preinstalled) I uninstalled McAfee (trial version was pre-installed), rebooted, ran the McAfee Consumer Products Removal Tool, rebooted again and Windows Defender has regularly autoupdated without incident and is working okay.

For further information see the reply of quietman7 in

Problems starting Windows Defender in Windows 8/8.1/10 ?

See   Good article but ignore the ads!

And see
 Configure Windows Defender in Windows 8, 8.1 and 10 


MVP Consumer Security 2014-2016
Windows Insider MVP 2016-2018

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Sorry this didn't help.



Thank you so much for the reply.

I found the answer to my questions regarding this error. It seems that Windows was updating in the background and WD was part of the update.

After Windows finished updating, which took more than two hours, and the standard reboot was done, Windows Defender happily, merrily and automagically started to work again. This is great.


This situation brings another question:

1. Why is a crucial Windows piece of software disabled during the update? During this blackout period, whatever it is, a PC is potentially exposed to virus infection.

I am happy with Windows and Windows Defender again! :)

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Sorry this didn't help.

Question Info

Views: 862 Last updated: May 15, 2018 Applies to: