John Celano start with this:

The iMac (27-inch, Late 2009) 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.

LED Functions

LED #1

• Indicates that the trickle voltage from the power supply has been detected by the main

logic board. This LED will remain ON whenever the iMac is connected to a working AC

power source. The LED will remain on even when the computer has been shut down or

put to sleep. The LED will turn off only if the AC power source is disconnected or the power

supply is faulty.

LED #2

• Indicates that the main logic board has detected proper power from the power supply

when the computer is turned on and that logic board voltage regulators are functioning

properly. This LED will be ON when the computer is turned on and the power supply is

working correctly.

LED #3

• Indicates that the computer and the video card are communicating. This LED will be ON

when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON

and you heard the startup sound, but LED 3 is OFF, then the video card might be installed

incorrectly or need replacement.

LED #4

• Indicates that the computer and the LCD panel are communicating. This LED will be ON

when the computer is turned on and video signal is being generated. If the LED is ON and

there is no image on the display, then the LCD panel, the LED backlight board, or the cables

between might be installed incorrectly or need replacement.

Let us know what you get on your iMac.