Q: I have a gray screen when I turn on my Mac and I can do a diagnostic test on it

Have you tried turning up your display brightness as mentioned in the HT document above.

Otherwise, try booting up in Safe mode by holding down the Shift key at startup. Does the same thing happen?

Also, try Recovery mode. Hold down Command and R keys at startup, choose Disk Utility and repair your startup disk.

Did it stay gray in Safe mode or only after you restarted in normal mode? What did Disk Utility say about your startup disk -- that it appeared to be OK? Also, you say you ran a diagnostics test, what was the result?

You might try the two other old faithfuls

OS X El Capitan: Reset your computer’s PRAM

Resetting the System Management Controller (SMC) on your Mac - Apple Support

Otherwise, you can reinstall the OS from Recovery mode or restore from backup.

Also, here's an article that deals with gray screen issues. It could be a peripheral cable.

http://macs.about.com/od/usingyourmac/qt/Mac-Stalls-On-Gray-Screen-At-Startup.ht m

Hi,

Found another Apple document about gray screens. OS X Yosemite: If you see a gray screen at startup