Chosen Solution
Your system has some onboard diagnostics LED’s which is located in the bottom right side of your logic board just out of sight by the chin plate and a strip of Black tape covering them. Follow this guide to Step 10. Using a keyhole flashlight and a pair of tweezers carefully lift off the tape covering the LED’s
Note the LED’s are read Right to Left! 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. Tell us what you discover
