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 was taken from here!

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 was taken from here!

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What do the beeps during the Power-On Self-Test (POST) mean?

This test runs whenever the computer is powered on after being fully shut down (the power-on self-test does not run if the computer is only restarted).

When a fault is detected during the Power-On Self-Test, you will not hear the normal startup chime. Instead, the system will beep as explained below. Macintosh Computers introduced after October, 1999 use a revised set of power on self-test beeps during startup.

1 beep = no RAM installed
2 beeps = incompatible RAM types
3 beeps = no good banks
4 beeps = no good boot images in the boot ROM (and/or bad sys config block)
5 beeps = processor is not usable

In addition to the beeps, on some computers the power LED will flash a corresponding number of times plus one. The LED will repeat the sequence after approximately a 5-second pause. The tones are only played once.

Note: In this case, a flash is considered to be 1/4 second or 250 ms or greater in length.

The information in this article was taken from here!

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What do the beeps during the Power-On Self-Test (POST) mean?

When a fault is detected during the Power-On Self-Test, you will not hear the normal startup chime. Instead, the system will beep as explained below.

1 beep = No RAM installed/detected
2 beeps = Incompatible RAM type installed (for example, EDO)
3 beeps = No RAM banks passed memory testing
4 beeps = Bad checksum for the remainder of the boot ROM
5 beeps = Bad checksum for the ROM boot block

Of course if you experience one of these beeps, Apple wants you to call your Apple-authorized service provider for additional assistance.

The power-on self-test resides in the ROM of the computer. Only computers that are based on the ROM-in-RAM (New World) technology, such as the iMac, Power Macintosh G3 (Blue and White), Power Mac G4 (PCI Graphics), PowerBook (FireWire) and PowerBook G3 Series (Bronze keyboard) will generate these beeps. This test runs whenever the computer is powered on after being fully shut down (the power-on self-test does not run if the computer is only restarted).

The information in this article was taken from here!

Information on Macintosh Machines from 1999 and newer will be posted tomorrow. You will find it under “Macintosh POST Beeps Defined – Part 2.”

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