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Hello. My iMac is not always going ON when i press Power Button, works only 20% of the time. The I have to unplug power and plug in again and try.. Diagnostic Led always ON when i plug it in, but when i press Power button then Leed 1 goes OFF and nothing happen. Sometimes when Led1 is ON and i press PowerButton it works normaly. Powersource was cleaned and PowerOn-Button is working. Whats the Problem? Logic Board, Power Supply? Thanks for your help.
I changed the Power Supply with no success. Then I pulled the screen off to see Diagnostic LED’s. I then shorted the Power Button pins (named PWR-BTN) on the logic board to test it. So I know the Power Button is working. PowerSupply is also working. SOLUTION: CHANGING MY HDD (already not original) with a new SSD solved the problem. So the problem was within the Disk Drive with the Boot sectors. I cloned it to the SSD with Disk Utility from iMac.
Sounds like a faulty power supply. LED 1 will be on if AC power is connected and if trickle voltage is going to logic board. If it’s off then all signs point to power supply. Link to Power Supply here
