I have a long out of warranty/AppleCare iMac12,2 that meets me with an Apple logo and progress bar on boot, and after a few minutes simply shuts off w/o booting to the system. I did try running Apple Hardware Test, and it simply freezes 3 minutes and 27 minutes into the test, every time I run it.
Booting in recovery and running Disk Utility on the volume gave me the message: "Error: Disk Utility can't repair this disk.Back up as many of your files as possible, reformat the disk, and restore your backed-up files."
From this point I did try booting in single user and running fsck and it met me with an error that it could not verify the disk and did not proceed beyond this point.
Obviously the hard disk needs replacing. Should I be OK just replacing the hard disk or should I be running other tests to make sure that there are no other issues with the computer prior to replacing the disk?