Mid 2011 27” iMac won’t turn on diagnostics LEDs stay dark

@qa1ysllewbt5zkq start off with the things that don't cost money:

• First disconnect the power cord from computer, then reconnect it to reset the LED status, in case computer was correctly shutdown and AC was kept connected since

• Verify AC source

• Verify known-good power cord is connected

• Verify cable connection between AC inlet and power supply

• Verify cable connection between power supply and logic board

Usese a voltmeter and check if you have AC power on the Power supply. do not forget to check you EMI filter power plug (where the power cord plugs in) Make sure you have power to the board from that. Since you are checking on 110v / 220V AC voltages, you do want to use common sense and do not touch anything with you bare hands etc.

If all of that checks out okay and since you are having a new power supply, the issue could be your logic board. Remember it is the logic board that "senses " the voltage to turn the diagnostic LED's on. If so then it is up to you want you want to do. Either replace or try to fix the logic board.