Introduction
The fan duct directs air across the heat sink on the logic board.
Tools
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Lay the iMac display-side down on a flat surface.
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Loosen the three Phillips screws securing the rear panel to the iMac.
 
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Lift the rear panel slightly near the bottom of the iMac.
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Pull the rear panel toward yourself and remove it from the iMac.
 
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                            Conclusion
                          
                          To reassemble your device, follow these instructions in reverse order.
Some models (with light sensor) have only the two outboard screws.
Robert Huber -