If your MacBook, MacBook Air, and MacBook Pro has a Sleep Indicator Light (SIL), the following behaviors occur. These behaviors also occur on some iMac and Mac Pro models, though not all Macs have a SIL.
Power-On Self-Test (POST)
If the SIL flashes one or more times after you press the power button to turn on the computer, and the pattern repeats after a brief pause, the computer may not have successfully passed POST.
- If your computer encounters hardware issues during POST, the computer may use on a combination of audible tones and blinking SIL behavior to communicate POST issue codes.
- To determine the cause of a POST issue, observe the SIL blinking pattern and refer to Intel-based Mac Power On Self Test RAM error codes.
When you press the power button to turn on the computer, the SIL will also turn on. As the computer starts up, the SIL stays on until the processor (GPU) has initialized and the display turns on. When the GPU initializes properly, the display will turn on and the SIL will turn off simultaneously.
Running and display sleep
While the computer is started and running, the SIL remains off. If the computer enters Display Sleep while running (the display turns off but the computer remains running), the SIL turns on and does not blink. It remains like this until the display turns back on, at which time the SIL turns off again.
You can enable Display Sleep via Apple menu > System Preferences > Energy Saver, or by pressing Control-Shift-Eject.
When the computer is in the Computer Sleep state, the SIL pulsates on and off slowly. You can enable Computer Sleep via Apple menu > System Preferences > Energy Saver.
On notebook computers, if the battery is completely depleted and the computer goes into Safe Sleep, the SIL doesn't illuminate at all.
When you choose the Shut Down command, OS X first turns off the GPU. This causes the display to turn off. When the display turns off, the SIL turns on. During the few seconds of shutdown, but before power is removed from the main logic board, the SIL remains on and will not blink. When the computer is fully shut down, the SIL turns off.
- You can use the SIL as a diagnostic visual to determine whether the computer is getting power at startup.
- If the SIL of a MacBook Air (Original, Late 2008, or Mid 2009) flashes five times when starting up or after waking from sleep, and the pattern repeats after a brief pause, the computer's battery may require service.