How to update your firmware, reset the SMC, and reset the NVRAM (or zap the PRAM), and what they do. Updated for High Sierra.
SMC
Safe mode, or Recovery? Which type of Recovery? How to engage all those useful modes at startup, and when shutting down.
Shutdown cause codes, wake reasons, ACPI, and sleep states – diagnosing problems from hardware events.
Can your Mac control sleep of its display, disks and system independently of one another? Many can’t.
These store key information for your Mac. Resetting the SMC and NVRAM is no panacea, but can transform an apparently dying Mac.
All the useful key combinations, arranged by function rather than keys.
What to do when your Mac won’t start up, and is acting dead, or at least mortally wounded.
After an interrupted firmware update, my iMac refuses to start up. What can I do?
The Eclectic Light Company 