Windows Defender can safeguard your system from malware by scanning the files and remediating the threats.
It is not an uncommon problem.
Searching on Google, you will find that lots of users are troubled by it.

Should this expectation be forlorn hope?
answers.microsoft.com
Now lets see how to address the engine unavailable issue.
Here are several feasible corresponding solutions to deal with the problem.
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Fix 1.
Here you’re free to follow the steps below to restart the Security Center service.
PressWindows + Rkey combinations toopen the Run window.

Typeservices.mscin the input box and press theEnterkey on your keyboard.
Scroll down to find and right-clickSecurity Centerand chooseRestartfrom the context menu.
Now you’re free to open Windows Defender to check whether the engine unavailable error has gone.
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Are Windows Defender exclusions not working?
This article offers you several useful tips to help you fix it.
Fix 2.
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Fix 3.
Modify the Registry Key
Note:Windows registry is very important to your machine.
Any wrong operations to the registry could damage your Windows system causing your PC unbootable.

So, before doing the following steps, double-check you havebacked up the registry.
PressWindows + Rkeyboard shortcuts to open Run.
In the text box, typeregeditand pressEnter.
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SelectYesin the User Account Control window.
In the top address bar, navigate to the following location:
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender
Step 4.
In the right panel, double-clickDisableAntiVirusand set its value data to0.
After that, clickOKto save the change.
ChangeDisableAntiSpywarevalue data to0by referring to the same steps above.
Fix 4.
Fix 5.
Then you might access and use it to protect your system just like you did for the first time.
Virus attacks could lead to data loss.
Torecover virus-deleted files, you could useMiniTool Power Data Recovery, a piece offree data recovery software.
without paying a penny.