I've changed the machine you selected to what I think is the correct one. The diagnostic LEDs on most of these are just to the right of the RAM slots. Sometimes there is a piece of black electrical tape extending from the bottom of the logic board and covering these LEDS. Just remove it. As you can see, this models entrance is through the front screen rather than opening the back. Much more difficult than the older G5 iMacs. The toughest part are the four screws that hold the LCD in place. Best tool is a highly magnetized torx driver to remove and especially replace these recessed screws. Let us know what the LEDs show. Note: the light diagnostics are different for Intel machines as opposed to the G5 lights.