Never mind, found the answer here on pg 29...
http://tim.id.au/laptops/apple/imac/imac...
"Diagnostic LEDs
The iMac (27-inch, Mid 2010) computer has four built-in diagnostic LEDs on the main logic board that can help you to troubleshoot the computer. The LEDs are located on the bottom left edge of the logic board (under a piece of black mylar tape) and can only be seen when looking through the lower vents of the enclosure. Refer to the next page for a close up graphic of the troubleshooting LEDs.
Location of Diagnostic LEDs
The LEDs are located under the mylar tape, on the bottom, left corner of the logic board. They can only be seen when looking through lower vents of rear housing"
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