@raxxx I am certain you at least already tried a SMC reset. If not, start with that. Next, let's look at the diagnostic LED's and when they should work and possibilities why they are not. For now, we'll stick just with LED 1
Diagnostic LEDs
LED 1
Indicates that the trickle voltage from the power supply has been detected by the main logic board. This LED will turn on when you connect the iMac to a working AC power source. The LED will remain on as long as the computer is on or asleep.
When the computer has been shut down correctly, LED 1 behavior may differ:
If a startup event is scheduled in System Preferences/Energy Saver, then LED 1 will stay on after a correct shutdown.
If no startup event is scheduled in System Preferences/Energy Saver, then LED 1 will turn off and will stay off as long as the power cord is kept connected and an AC power source is present. Disconnecting the power cord and plugging it back in will turn this LED back on, even if the computer is still off.
After disconnecting and reconnecting the AC power source, ***this LED could remain off***:
***If the AC power source is missing or disconnected.***
If the ***logic board is disconnected from the power supply*** or the AC receptacle. If the ***power supply board is faulty***. [br]
and here is the sequence for LED 1 to light up.
LED Startup Sequence LED 1 = Power is available.
If no LED is visible:
Disconnect the power cord from the computer and wait 15 seconds to reset the power supply and LED status. Reconnect the power cord and check the LED status again.
Verify the AC source.
Verify that a known-good power cord is connected.
***Verify the cable connection between the AC inlet and the power supply. Verify the cable connection between the power supply and the logic board. Verify the power supply.*** [br]
For now, I suggest you focus on the power supply. You told us you replaced the power supply but you did nto tell us if you checked that it A. ***gets power*** from the outlet and B. it does ***have power for the logic board***. If you check the functional overview for your iMac you see that the no power issue is focused around the connection from the power supply to the logic board [document|41445]
[image|3586357]
You can check the voltage on pin 4 and pin 5 and let us know what you get. Also, you may consider taking a couple of large, well-focused pictures of your logic board and post those with your Question. That way we can see what you see and hopefully guide you further. [guide|21499]
Do not forget to check the power button :-)
Last but definitely not least, go through the Apple recommended [document|41446] Let us know what you find out.