Macintosh Power-On Self Test

This document is an excerpt from the Apple Knowledgebase

If you have never experienced the problem of diagnosing POST failures, consider yourself fortunate. While computer components normally outlast their usefulness, they do occasionally fail to the point where a machine cannot boot. The Power-On Self Test is the initial hardware test a machine performs to check certain system components such as the RAM and video card. If any of these critical components fails the POST, you may hear a series of beeps. The number of tones indicates the source of the problem.

Pre-1999 Macintosh Computers

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).

If a fault is detected during the test, you will not hear a normal startup chime. Instead, the system will beep as explained below. If you experience one of these beeps, you should call your Apple-authorized service provider for additional troubleshooting assistance.

• 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

Post-1999 Macs

Products introduced after October, 1999 use a revised set of power on self-test beeps during startup. This article describes each one.

The power on self-test resides in the ROM of the computer. This test runs whenever the computer is turned on after being fully shut down (the power-on self-test does not run if the computer is only restarted).

If a fault is detected during the test, you will not hear a normal startup chime. Instead, the system will beep as explained below. If you experience one of these beeps, you may call your Apple Authorized Service Provider for additional troubleshooting assistance.

• 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.

Intel-based Mac Power On Self Test RAM error codes

Intel-based Macs such as the iMac and Mac mini computers introduced in 2006, 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.