I created a scheduled task to run a quick scan on all my computers which does seem to run pretty reliably. Although after a major update it sometimes may not. If so I delete the scheduled task and create a new one and that seems to work. What's funny is I have disabled the scan as part of maintenance yet that still runs too. Though not reliably. So on some days I get multiple scans run.
After both an in place upgrade to 16299.19 on a Home laptop and a clean install on a PRO desktop, I found while the scan task still runs fine with the usual 'The operation completed successfully, (0x0)', the other Defender task to update the definitions - '../MpCmdRun.exe -signatureUpdate' - is now occasionally returning this lone '(0x2)' message.
Oddly when I ran the task manually as a test, it failed every other attempt, as if it were getting a busy signal. However checking the last few days, it's been (0x0) fine.
I have notice Windows still feels the need to run a scan on it's own whenever it feels like it. Usually after some new definition signatures have come in via the normal daily WU cycle. purely speculating that it thinks they've been sitting around untested too long as I've noticed concurrently the Defender tray icon gets the yellow '!' warning sign. And I have the Defender's scans set to notify me in the Action Window. where at that the end of the week I'll get a 'Scan Summary', often noting as many as 9 scans in the 7 day cycle.
Of course this was all a much bigger issue for me in the earlier versions of Windows when even the 'quick' scans were taking 10 times longer to run than now which is but a minute or two.
Thought I would update....
I'm on Ver 1903 now and still the windows defender schedule scan (in task scheduler) has not run.
Still getting the 0x2 error. The other 3 tasks have run correctly.
Now 3 years since I first posted.
MS have got to be joking....
Ever since the Anniversary Update broke Defender's automatic scan, Microsoft's been telling users it's not important because Defender has real-time protection. MS's suggestion -- run a manual scan or create a custom scan task. For reasons known only to MS, fixing this has never been a priority and by now, it's probably not even a consideration.
I seem to recall that starting earlier this year, MS began to change some paths in Defender and that's been creating some 0x2 errors for custom scan tasks. The following linked thread is old, but posts 13 through 16 are new and corroborate your experience. Schedule Scan in Windows Defender Antivirus in Windows 10
I just run a manual scan when I feel like it.
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