Intel-based Macs such as the iMac, Mac mini, MacBook, MacBook Pro, and Mac Pro rely on a combination of tones and blinking LEDs to display Power On Self Test (POST) error codes.
If the computer detects no SDRAM (Synchronous Dynamic Random Access Memory, also referred to as RAM) or the RAM installed does not meet the appropriate specifications, the screen will remain black but the power LED on the front of the computer will blink once per second to signal the error. This error condition may be due to physically damaged RAM, installing the incorrect type of RAM, or not having RAM installed.
Some RAM may appear to pass the Power-On-Self-Test (POST) but still cannot be used by the operating system. In this case, the computer will display a gray screen, sound three tones and blink the power LED on the front of the computer three times, pause, and repeat the blinking until the computer is turned off.
The solution to both of these situations is to first re-seat the memory and test the computer again. If the memory fails the POST test again, try memory that has been verified to work correctly on another system (i.e., “known-good”) or order new memory from the Apple Store.
Products Affected: MacBook Pro, MacBook, Mac Pro, Mac mini, iMac, MacBook Air
The information in the first part of this article came from About Mac Power On Self Test (POST) RAM error codes!
Some newer Macintosh models only use tones to indicate error conditions.
The iMac (Mid 2007) and later model iMac computers, unlike other Macintosh computers, do not use the power LED to indicate there is an issue. iMac (Mid 2007) and later model iMac computers use tones instead.
Products Affected: iMac (20-inch, Mid 2007), iMac (24-inch Mid 2007), iMac (24-inch, Early 2008), iMac (20-inch, Early 2008)
Tone(s) – Description
1 tone, a 5-second pause, repeat – No RAM installed
3 successive tones, a 5-second pause, 3 successive tones – RAM does not pass data integrity check
1 long tone when holding down the power button – Firmware upgrade in process.
3 long tones, 3 short tones, 3 long tones – Firmware restoration from CD in process.
The information in the second part of this article came from About Mac startup tones!