Until October 2016, Macs made sounds when starting up, to indicate their health or serious problems. On Intel-based models prior to the MacBook Pro 2016, the following sounds are possible:
- normal startup chime – memory and ROM appear good, normal startup will continue
- a single tone repeated every 5 seconds – no RAM has been detected
- three tones followed by a 5 second pause, repeating – installed RAM has failed initial testing
- three long, three short, and three long tones (the inverse of Morse SOS) – EFI ROM is corrupt and is being recovered
- one long tone when you hold the power button – EFI ROM update is in progress (Macs before 2012).
Much older PowerPC and Classic Macs are different again, and some flash the display rather than making error tones.
The great majority of problems with EFI ROM corruption occur when an EFI update is interrupted. Spontaneous corruption suggests that something even more serious is going on, and needs full hardware diagnostics.
EFI ROM updates only occur now as part of a macOS update. All models which are upgraded to run High Sierra receive a firmware update as part of that installation. These are usually listed in the Installations item in System Information, which will also give full details of the installed version.
If something goes wrong with an EFI ROM update, or the Mac loses power during the update, the Mac can be left with corrupt firmware, and may refuse to start up properly. If that happens:
- more recent models should automatically recover the EFI ROM after sounding the — … — tone,
- MacBook Pro 2016 models will not make any tones, but should also automatically recover the EFI ROM,
- old models, prior to about 2008-10, may need to be started up from a Firmware Restoration CD to restore the EFI ROM.
Once a recent model has sounded the tones indicating it has entered EFI ROM recovery, a progress bar should be displayed, during which recovery will take place automatically. Once complete, the Mac will restart normally and should function correctly again. If it does not, you can try resetting its SMC and NVRAM, as described here. A 2016 MacBook Pro should do the same, but in silence.
Older models require you to create a Firmware Restoration CD, provided by Apple in the form of a downloadable disk image, which you can then burn to CD with Disk Utility on another Mac. This process is detailed here.
If none of these help you start your Mac up successfully, you’ll need to get it to an Apple engineer, either through a Genius Bar or Apple support.
The latest MacBook Pro 2016 models with a Touch Bar introduce additional firmware, which is used by the processor which drives the Touch Bar. This has been updated as part of macOS Sierra updates, including 10.12.4. If that update does not complete properly, the Touch Bar firmware can be corrupted, which can leave your Mac cycling endlessly through crashing during startup, then restarting again – an infinite boot loop.
You should be able to break an infinite boot loop by entering Recovery mode, then repairing the startup volume. You may need to re-install macOS before the problem is fully repaired, though.
(Updated 15 September 2017 to cover High Sierra.)