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I have been trying to figure this out but can’t. The Mac is being quite unpredictable when it is powered on. I have removed the screen to get to the diagnostic LEDs and to change the internal battery. Basically, when power is plugged in LED 1 is lit up - that is the same no matter what. When the power button is pressed, some times LED 2 will flash on and off, sometimes it stays on for a few seconds and goes off and sometimes it stays on until I turn it off. The fan will kick in at full speed when the computer stays on for more than a few seconds. There was one time after removing the on board battery when it came on and chimed, then LED 3 was on. It was very dim but it was lit. haven’t been able to get that to happen since. I have tried removing and re seating the RAM and changed out the sticks without any change. Any Suggestions?
Sadly I’m suspecting your power supply has failed. Even though you have a surge suppressor it might have failed you. I would also recommend replacing the surge suppressor with a good quality unit or better yet getting a good UPS. For reference here’s a full breakdown on the 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 dedicated GPU are communicating. This LED will be ON when the computer is communicating properly with the dedicated GPU. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the logic board is in 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 cables from it might be installed incorrectly or need replacement. Here’s the needed part iMac Intel 27" (Late 2012-2020) Power Supply and here’s the guide to put it in iMac Intel 27" Retina 5K Display Power Supply Replacement
