Diagnostic LEDs
Depending on iMac model, there are either three or four built-in diagnostic LEDs on the logic
board that can help troubleshoot the computer. LEDs are located on bottom right edge of logic
board (under a piece of black mylar tape) and can only be seen when looking through lower
vents of rear housing.
LED Functions
LED #1
• Indicates that trickle voltage from power supply has been detected by 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 computer has been shut down or put to sleep. The LED will turn off
only if AC power source is disconnected or power supply is faulty.
LED #2
• Indicates that logic board has detected power from power supply when computer is
turned on. This LED will be ON when computer is turned on and power supply and voltage
regulators are working correctly.
LED #3
• Indicates that computer and video card are communicating. This LED will be ON when
computer is communicating properly with video card. If LEDs 1 and 2 are ON and you heard
the startup sound, but LED 3 is OFF, then video card might be installed incorrectly or need
replacement. Note: LED #3 is not present on models with integrated graphics.
LED #4
• Indicates that computer and LCD panel are communicating. This LED will be ON when
computer is turned on and video signal is being generated. If LED 4 is ON and there is
no image on display, then LCD panel, LED backlight board, or cables between might be
installed incorrectly or need replacement.
LED Startup Sequence
LED #1 = Power available.
If no LED is visible:
• Verify AC source
• Verify known-good AC cable is connected
• Verify cable connection between AC inlet and power supply
• Verify cable connection between power supply and logic board
• Verify power supply
LED #1 + LED #2 = Power available, and system is powered on.
If second LED is not visible when power button is pressed:
• Verify power button connection to logic board
• Verify power button functionality
• Verify cable connection between power supply and logic board
• Verify power supply
• Verify logic board
LED #1 + LED #2 + LED #3 = Power available, system is powered on, and video card found.
If third LED is not visible after power on:
• Verify that MXM video card is seated properly
• Verify logic board
LED #1 + LED #2 + LED #3 + LED #4 = Power available, system is powered on, video card
found, and internal LCD found.
If fourth LED is not visible after power on:
• Verify cable connections between LCD panel and logic board
• Inspect LCD display cables for cable damage
• Verify external video functionality, and according to result check the following items:
-If external display works then verify/replace LED backlight board
-If external display works then verify/replace LCD panel
-If external display does not work verify/replace logic board
Deep Dive
Check Result Action Code
1. Verify boot chime present and fans running when system powered ON. (Reset SMC and clear PRAM if necessary for proper boot up.)
Yes
Power ON self test OK. Boot sequence started.
Go to step 2.
No
Go to Won’t Start Up symptom flow.
2. Verify if image is visible on built-in LCD panel.
Yes
Video present. Verify system functionality and return to user or jump to appropriate troubleshooting flow.
No
Go to step 3.
3. Connect supported external display. Verify if image appears on external display when system is booted.
Yes
External display detected by system. Video circuitry on logic board functional.
Inspect LVDS cable connection to back of LCD panel. Reseat and test again. If issue persists, go to Backlight Issue/No Backlight symptom flow.
No
Go to step 4.
4. Pivot enclosure to access lower intake vent, and locate diagnostic LEDs on edge of logic board. Verify if LED #3 turns on shortly after system powers on.
Yes
Logic board communicating with video card. Go to step 5.
No
If LED #3 does not turn ON
-If system has a separate video card, reseat video card and retest, then replace video card and repeat step 4. If issue persists, replace logic board.
-For a system without video card, replace logic board.
5. Locate diagnostic LEDs on logic board. Verify if LED #4 turns on shortly after system powers on.
Yes
Logic board communicating with LCD panel. Go to Backlight Issue/No Backlight symptom flow.
No
Replace LVDS cable.
Go to step 6.
6. Locate diagnostic LEDs on the logic board. Verify if LED #4 turns on shortly after system powers on.
Yes
Video controller functional and communicating with LCD panel. Go to Backlight Issue/ No Backlight symptom flow.
No
Video controller unable to communicate with LCD panel. Reseat cables first. If issue persists, replace LCD panel.
Retest.
Before replacing any board i.e. logic board etc., let us know what you get/find.
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