Random Shutdown
What causes random shutdown/restart 21.5" 2017 iMac
iMac 21.5″ 4K, macOS 11.6
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Hello Lamoreux,
To help resolve issues with your iMac restarting, let's start with the steps in this article:
If your Mac restarted because of a problem - Apple Support
Unexpected restarts are usually caused by software installed on your Mac, or by devices connected to your Mac. If the issue causes your Mac to restart every time it attempts to start up, your Mac might eventually shut down. Use the steps below to check the software and hardware on your Mac.
Check your software
• If the issue is caused by software on your Mac, one of these steps might help:
Install all available software updates.
• If your Mac suspects that a particular app caused the restart, it might ask whether you would like to move the app to the Trash. Click Move to Trash, then contact the software developer to see if a software update is available.
• Use safe mode to try to isolate the cause of the issue.
• Reinstall macOS. If the issue continues after reinstalling macOS, check your hardware.
Best Regards.