The following is an overview of the startup sequence, error conditions, and behaviors for all Intel-based Macintosh systems.
Note that some newer Macintosh models only use tones to indicate error conditions. See this article for more information.
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When upgrading the SMC (System Management Controller) firmware on an Intel-based Mac, you experience issues if power was interrupted during the update process, or if the firmware update installation did not work for other reasons. Issues may occur when starting up, displaying video, going to sleep, or there may be fan noise issues.
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