

Software conflicts (possibly with another antivirus program), corrupted registry, etc. Factors contributing to these issues may include malware infection,

Sometimes, due to various factors, Windows Defender do not start automatically when Windows starts and when Windows Defender is started manually via Action Center it may display an error code. This wiki describes some methods that can be used to fix the problem.

Windows Defender in Windows 8/8.1/10 sometimes do not start automatically and may report an error when started manually.
