iMac starts and powers off before getting all the way on
iMac starts and then powers off about 3/4 the way thru power on?
iMac Line (2012 and Later)
Question marked as
Top-ranking reply
Unfortunately that is generally caused by a hardware issue.
Try Safe Mode, as per > https://support.apple.com/en-us/HT201262
And macOS Recovery, as per > https://support.apple.com/en-us/HT204267
Then the Apple Diagnostics, as per > https://support.apple.com/en-us/HT202731
Sadly if none of those get it going or help, see > https://support.apple.com/en-us/HT204267