Before you remove the power supply from the system test the system out and review the onboard diagnostic LED’s
What does the onboard diagnostic LED’s telling you?
- 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 video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or 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 inverter might be installed incorrectly or need replacement.
No LED's does imply the system is not getting power! Either from the outlet, power cord or the systems power connection and lastly the power supply could be damaged!
Go to a local hardware store and get an 3-Wire Receptacle Tester so you can make sure the outlet is correctly wired and is properly grounded. You'll also need to inspect the breaker panel to make sure the grounding line is properly secured to the metal water line or a grounding rod. Often overtime these degrade with corrosion.
Check your power strip / surge suppressor make sure the onboard breaker has not popped or switched off. Often times when the breaker has popped a short occurred. Next try replacing the power cord to your system. You might want to invest in a good UPS or at least replace the surge suppressor as over time they do loose their effectiveness. If after all of this testing you can’t locate any issue is when I would conclude the power supply has failed. Do keep in mind a surge or a lighting hit could also have damaged your logic board as well, but you won’t know that until you’ve replaced the power supply