Patrick, right now it appears like a video card issue, but check this first:"
Diagnostic LEDs
There are four built-in diagnostic LEDs on the logic board that can help troubleshoot the computer. LEDs are located on the front center of the logic board, above the IR board, to the left of the SMC reset button.
LED Functions
LED #1
• Indicates that the trickle voltage from power supply has been detected by main logic board.
This LED will turn ON when you connect the iMac to a working AC power source. The LED will remain ON as long as the computer is ON or asleep.
• When computer has been correctly shutdown, the LED#1 behavior may differ:
• - If a startup event is scheduled in System Preferences/Energy saver, LED#1 will stay ON after a correct shutdown,
• - If no startup event is scheduled in System Preferences/Energy saver, LED#1 will turn OFF and will stay OFF as long as AC cable is kept connected and AC power source is present.
Disconnecting the AC cable and plugging it back will turn back this LED ON , even if computer is still off.
• After disconnecting and reconnecting the AC power source, this LED could remain OFF if the AC power source is missing or disconnected , if the logic board is disconnected from the power supply or from the AC receptacle, or if the power supply board is faulty.
LED #2
• Indicates that computer is turned on. This LED will be ON as long as computer is turned on (but is not asleep) 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 the backup battery (on back 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 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.
LED Startup Sequence
LED #1 = Power available.
If no LED is visible:
• First disconnect the AC cable from computer, then reconnect it to reset the LED status, in case computer was correctly shutdown and AC was kept connected since
• 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 the MXM video card is seated properly
• Verify if boot chime is present and fans are running when powered ON (reset SMC and PRAM, reseat or check backup battery if necessary for proper boot up):
If POST boot chime is not heard, go to Won’t Start Up symptom flow,
If POST boot chime is heard, go to No Video symptom flow.
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 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"
Let us know what you find after those tests. Hope this helps, good luck.
1.1mil 
893 
2.3mil 