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I have a 27” iMac mid 2011 and suddenly the display went blank. I checked the onboard diagnostics and LED 3 and 4 are off. I purchased a new graphics card and logic board but now the same problem. LED 1 and 2 turn are on and LED 3 and 4 are off. What is the solution ?
Information directly from the source: 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 the backup battery (on front of logic board) may need to be reseated, or the video card might be installed incorrectly or needs replacement. 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 LCD and logic board or backlight board might be installed incorrectly, or need replacement.
