Chosen Solution

After pressing the power button on my iMac Intel 27" Retina 5k Display, the following happens:

  1. 3 x green LED’s light up continuously (I’m looking throught the RAM tray)
  2. I hear the Hard Drive spinning up and the main fan sounds like it comes on
  3. There is no Bios Beep and no screen output
  4. After about 5 seconds, the Mac powers itself down
  5. The Mac keeps restarting itself again after 5 seconds or so I tried swapping out the RAM with a known working set but that hasn’t helped. I can however get a Bios Beep if I remove all the RAM. Other than that I don’t think there is anything else I can try without taking the screen out. And even then I wouldn’t know what to look for. :) Can anyone help me please?

I’m not sure what you are seeing through the RAM tray. The onboard diagnostic LED’s are on the front side of the logic board behind the display panel (you need to remove the display assembly) The only thing I can think of is you are seeing LED’s built into the SO-DIMM’s. Some vendors add LED’s to allow you to trick-out your system (clear cases) World’s First RGB DDR4 SO-DIMMs! Personally I think they are a waste in an iMac as you can’t see them! Here’s the location of the diagnostic LED’s

And here’s a deeper look on what they mean

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. LED4 won’t light with the display disconnected. Apple uses a frame to hold the dimply to the side and extension cable set. Now with a better idea what you need to checkout I would first put back the original RAM as the RAM you got is likely the wrong type before you go in. Locate your system from this listing to get the details of what you need for RAM specs EveryMac - iMac’s to check what you do have if different you’ll need to get the required SO-DIMM’s

Quite a stumper. My only thought is that I know Macs can be really picky about ram. If it were me, I’d put only one module in at a time, swapping them out to see if any one of them worked at all. And I’d check one more time on which specs and brands are known to work. I’d take these steps just because they’re simple, cheap and possible to do immediately. Good luck!