IMac won’t boot - disk utility found a problem

I have a late 2013 iMac currently running OS X 10.13.3. It won’t boot up past the Apple logo screen, turning itself off when the progress bar reaches about 2/3 full. I have booted it up in recovery mode and used Disk Utility to run first aid on the Fusion Drive hard drive (which only has one volume, “Macintosh HD”). It tells me that “First Aid process has failed. If possible back up the data on the volume.”

In the details, at the end it says:

Performing fsck_hfs -fy -x /dev/rdisk3

Checking Journaled HFS Plus volume.

Checking extents overflow file.

The volume Macintosh HD could not be verified completely.

File system check exit code is 8.

Restoring the original state found as mounted.

Problem -69842 occurred while restoring the original mount state.

File system verify or repair failed.

Operation failed…

When I run Firat Aid on the Macintosh HD volume I get the same error message, but the details say don’t have the line that begins with “Problem -69842”

I also have an OS X Base System Disk Image. When I run first aid on that image, it completes successfully. However, I am still unable to boot.

Does anyone have any ideas what my next steps should be?