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Power On Self Test (POST) in iMac 21.5 inch, 2009

Intel-based Mac computers such as the iMac rely on a combination of tones to indicate Power

On Self Test (POST) error codes.

• If the computer detects out-of-specification or no SDRAM, the screen will remain black
but the computer will beep. This error condition may be due to physically damaged RAM,
installing 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 repeat tones until computer is turned off.

• The solution to both of these situations is to first re-seat memory and test computer again.
If memory fails POST again, remove all installed memory and test by installing one by one
each memory module that has been verified to work correctly on another system (i.e.,
“known-good”) or order new memory.

• A sequence of tones heard at startup or a no video symptom may also be fixed by
temporarily removing/replacing the backup battery.
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