This is from an Apple Certified repair person: Installed incorrectly or need replacement. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be This LED will be ON when the computer is turned on and video signal is being generated. Indicates that the computer and the LCD display panel are communicating.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. This LED will be ON when the computer is communicating properly with the video card.
This LED will be ON when the computer is turned on and the power supply is working correctly. The LED will turn off only if the AC power source is disconnected or the power supply is faulty. The LED will remain on even when the computer has been shut down or put to sleep. This LED will remain ON whenever the iMac is connected to a working AC power source.
