Chosen Solution
Recently picked up a really cheap iMac, first time it booted from USB perfectly (no hdd included). upon loading high-Sierra installer the screen went white, I thought it froze so I turned it off using the power button.. Tried to turn it back on and got nothing, no display, no chimes. However I can hear the hdd spinning, and the fans going as well. Upon opening it, LED 1&2 are illuminated, but 3&4 are not. Is this a symptom of a bad graphics card? Or is there other stuff I should check first?
Here’s a breakdown of the onboard diagnostic LED’s: LED 1 - Indicates that the trickle voltage from the power supply is detected by the main logic board. This LED will remain ON while the iMac is connected to the AC power. The LED will remain on even when the computer has been shut down or put to sleep. The LED will turn off only if the AC power is disconnected or the power supply is faulty.LED 2 - Indicates that the main logic board has detected proper power from the power supply when the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly.LED 3 - Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.LED 4 - Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement. So as you can see a failed GPU board is a strong contender! Try this take the card out clean off the old thermal paste refreshing it and replace the VRAM chips thermal pads see if that helps. Some people have baked their cards to get a little more time out of them. Replacing it gets a bit iffy these days as AMD has long since stopped production and Apple no longer services this iMac, so all you’ll find are used boards.
