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.
Indicates that the main logic board has detected proper power from the power supply when • the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly.
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.
Indicates that the computer and the LCD display 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 LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement.
6. LED #4 should be On when the video card and LCD display have communicated properly to produce a video image. Is LED #4 On or Off?
LED # 4 is On: The video card and LCD display have communicated to LCD video. In this case, there is a problem with the backlights in the panel or the inverter. First, make sure the four inverter cables are fully inserted into the panel connectors and check that the four wires have not been pinched. Next, with the computer powered on, turn the display at an angle to you and look closely at the display. It may be necessary to dim the room lights. Is the display completely black, or can you see a dim light or purple glow to the display?
Completely black: Replace the inverter.
Dim light or purple glow: The inverter and panel backlights are functioning. Replace the LCD display panel.
LED # 4 is Off: The video card and logic board are unable to communicate to generate video. Replace the video card. Check all your connections again.