My iMac has been shutting down abruptly

An "anti-virus scan" is a waste of time.

Run Apple Hardware Testto ensure its fans are operating correctly.

Apple Hardware Test is for Macs produced prior to June, 2013. Later Macs use Apple Diagnostics.

After it restarted, did you see a dialog box resembling this one:

If so, next time it occurs click Report...

Before you send it to Apple, copy the text of the report. Paste it in a reply.

Remove or obscure any personal information, should it appear.

Back up all data immediately as your boot drive may be failing.

This diagnostic procedure will query the system log for messages that may indicate a hardware fault. It changes nothing, and therefore will not, in itself, solve your problem.

If you have more than one user account, these instructions must be carried out as an administrator. I've tested them only with the Safari web browser. If you use another browser, they may not work as described.

Triple-click anywhere in the line below on this page to select it:

syslog -k Sender kernel -k Message CReq 'GPU D|I/O|nspace-h|n Cause: -|timed? ?o' | tail | open -ef

Copy the selected text to the Clipboard (command-C).

Launch the Terminal application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad. Click Utilities, then Terminal in the icon grid.

Paste into the Terminal window (command-V).

The command may take a noticeable amount of time to run. Wait for a new line ending in a dollar sign (“$”) to appear.

A TextEdit window will open with the output of the command. Normally the command will produce no output, and the window will be empty. If the TextEdit window (not the Terminal window) has anything in it, post it — the text, please, not a screenshot. The title of the TextEdit window doesn't matter, and you don't need to post that.

Hey, I have been having the same issue and followed your instructions and this is what my text edit window displayed.

Nov 6 15:33:02 julians-imac kernel[0] <Debug>: IOSCSIPeripheralDeviceType00::setPowerState(0xffffff802b76ce00, 3 -> 0) timed out after 100933 ms

Nov 6 17:13:06 julians-imac kernel[0] <Debug>: IOSCSIPeripheralDeviceType00::setPowerState(0xffffff802bee6e00, 3 -> 0) timed out after 100931 ms

Nov 9 15:18:36 julians-imac kernel[0] <Debug>: IOSCSIPeripheralDeviceType00::setPowerState(0xffffff8045c43200, 4 -> 0) timed out after 100838 ms

Nov 11 06:09:01 julians-imac kernel[0] <Debug>: IOSCSIPeripheralDeviceType00::setPowerState(0xffffff80463b1000, 3 -> 0) timed out after 101007 ms

Nov 11 13:41:20 julians-imac kernel[0] <Debug>: IOSCSIPeripheralDeviceType00::setPowerState(0xffffff805a38bc00, 4 -> 0) timed out after 100998 ms

Nov 11 14:10:19 julians-imac kernel[0] <Debug>: IOSCSIPeripheralDeviceType00::setPowerState(0xffffff805a66d600, 4 -> 0) timed out after 101005 ms

Nov 11 15:25:24 julians-imac kernel[0] <Debug>: IOSCSIPeripheralDeviceType00::setPowerState(0xffffff805a889200, 4 -> 0) timed out after 101003 ms

Nov 11 17:15:00 julians-imac kernel[0] <Debug>: IOSCSIPeripheralDeviceType00::setPowerState(0xffffff805a8a0400, 4 -> 0) timed out after 100997 ms

Nov 12 08:33:43 julians-imac kernel[0] <Debug>: IOSCSIPeripheralDeviceType00::setPowerState(0xffffff805ac52200, 4 -> 0) timed out after 101002 ms

So I was having this issue constantly after buying my late 2012 27 inch imac. After some googling and a completely different topic I unchecked the box for "put your disc drives to sleep when possible" and haven't had any troubles with it shutting down. So not sure what the issue is.