Chosen Solution
This unit does incomplete boot, its working in Safe mode, but not in normal booting. According to Apple tech support, possible problem are memory, hard drive, logic board replace hard drive. I did replaced accordingly, one by one, but no luck. The last thing I did was replacing the logic board assuming this the end of the problem, however, the problem remain the same. just let you know that OS installation was complete and restart automatically, however booting failed, didn’t go to login screen, the screen remains gray, and no sound or chime. The diagnostic LED’s lit accordingly. I don’t know what else to do. Needing help to resolve the problem. Update (11/22/2018) see the screen on my mac
If it boots in Safe Mode, disregard everything the Genius told you. Your problem is in the system extensions. On this old of a machine, I would insert my installation DVD start up by holding down the C key. Reformat the drive using Disk Utility (better would be to wipe the drive drive with a single pass write zeros option to map out any bad blocks). Now format the drive GUID and Mac OS X extended Journaled. Now install your original system.
I have a similar problem. iMac shows a gray screen with an apple logo. In safe mode it loads, but there are artifacts on the display.
