@kamha let us know what board your computer has so we can follow you along during your diagnsotics. Also here is what apple says reasons for LED 1 being off could be. Check on your AC side first before going to deep into any logic board issues.
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, LED 1 will stay on after a correct shutdown.
If no startup event is scheduled in System Preferences/Energy Saver, 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.
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.
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