Verify that video card is seated properly. LED #1 + #2 + #3 = Power available, computer is powered on, and video card found. Verify power button cable connection to logic board. If second LED is not lit when power button is pressed: LED #1 + #2 = Power available, and computer is powered on. Verify cable connection between power supply and logic board. Verify cable connection between AC inlet and power supply. Verify known-good power cord is connected. Disconnect the power cord from computer, then reconnect it to reset the LED status. 
Or backlight board might be installed incorrectly, or need replacement. Image on display, then LCD panel, LED backlight board, or cables between LCD and logic board This LED will be ON whenĬomputer is turned on and video signal is being generated. Indicates that computer and LCD panel are communicating. Startup sound, but LED 3 is OFF, then the backup battery (on front of logic board) may need toīe reseated, or the video card might be installed incorrectly or needs replacement. This LED will be ON whenĬomputer is communicating properly with video card. Indicates that computer and video card are communicating. Is not asleep) and power supply and voltage regulators are working correctly. This LED will be ON as long as computer is turned on (but Power supply or from the AC outlet, or if the power supply board is faulty. The AC power source is missing or disconnected, if the logic board is disconnected from the
After disconnecting and reconnecting the AC power source, this LED could remain OFF if. 
OFF and will stay OFF as long as power cord is connected and AC power source is present.ĭisconnecting the power cord and plugging it back will turn back this LED ON, even if
- If no startup event is scheduled in System Preferences > Energy Saver, LED#1 will turn. - If a startup event is scheduled in System Preferences > Energy Saver, LED#1 will stay ON. When computer has been correctly shut down, the LED#1 behavior may differ: Remain ON as long as the computer is ON or asleep. This LED will turn ON when you connect the iMac to a working AC power source. Indicates that the trickle voltage from power supply has been detected by main logic board. Or resposition the audio cable in order to view the LEDs. Note: The audio cable covers the diagnostic LEDs, so you will need to disconnect There are four diagnostic LEDs on the front center of the logic board that can help troubleshoot
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