Fixing NVRAM and SMC
Mac users of a certain age may well remember a fix known as ‘zapping the PRAM’ – using a quirky key combination at startup to fix myriad problems. Modern Macs feature something similar, called NVRAM (Non-Volatile Random-Access Memory). This, along with resetting the SMC (System Management Controller) can fix a variety of startup and charging issues.
If settings in NVRAM are corrupt, or SMC is out of sync, your Mac will exhibit a variety of weird behaviours. The Mac may take a long time to start, display in the wrong resolution, or its time zone may be erratic. The SMC controls fans, status lights, the keyboard backlight and other components.
To reset NVRAM, shut down your Mac, press the power button and as soon as you hear a chime hold ç+å+p+r. You’ll hear a second chime. Let go of the keys and the Mac will start normally.
Resetting the SMC is a last resort after all other troubleshooting fails. If your Mac’s symptoms match anything listed under Indicators at http://apple.co/1N7tsy1 then you should reset the SMC by following the instructions for your type of Mac on that page. The process differs between Mac notebooks depending on whether their battery is removable, and for desktops.