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how to LED Startup Sequence in iMac 2015

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
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