lexie huseyin,
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 the LCD panel, the LED backlight board, or the cables between LCD
and logic board or backlight board might be installed incorrectly, or need replacement.
If fourth LED is not visible after power on:
• Verify internal DisplayPort 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 the LED backlight board
If external display works then verify/replace the LCD panel
If external display does not work verify/replace the logic board
Since your iMac detects an external display, the Video circuitry on your logic board functional. Inspect DisplayPort cable to back of panel connection. Reseat and test again. If you still have trouble, I would focus on the backlight board. Shine a flashlight at an angle against the screen in a darkened room and see if you can make out any shades/shadows. If so I'd replace the backlight board before the panel.