Here’s another diagram, this time to help you diagnose and deal with a Mac which won’t start up into the Desktop and Finder.

At the left are your actions using your Mac’s controls (green). The series of purple boxes then details the sequence of screens which you should see. When a Mac fails to progress to the next screen in the sequence, then the other likely screens are shown in the centre column, with error screens shown in red. Additional information, such as audio signals, are shown in the next column, in pink. At the right are some of the most probable causes and some actions which you can take to address the problem.

Signals used to indicate problems detected during and shortly after POST are too complex to place on the diagram. They are:

Tones:

• single tone, every 5 seconds = no RAM detected
• three tones, five second pause, repeats = RAM doesn’t pass testing
• three long, three short, three long = EFI ROM corruption, Mac is in EFI ROM Recovery mode; watch recovery progress bar, then the Mac should restart automatically.

Power LED flashing:

• blinks once every second = no RAM detected, or RAM doesn’t pass testing
• blinks three times, pauses, repeats = RAM detected but can’t be used by macOS.

I hope that you find this useful. If you have any additions or corrections, please add them here as comments so that I can amend the diagram.