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The power supply puts out a trickle charge to light LED1. LED1 basically indicates that the power supply is plugged in and has power. If LED 1 is blinking when you press the power button your power supply is too weak to handle the current load of powering on the iMac or you have a bad connection in your wall socket. If your wall socket is good, then you need to replace the power supply. The diagnostic LEDs are the best way to determine what the source of the problem is in the iMac. LED #1 = Power available. If no LED is visible: • Verify AC source • Verify known-good AC cable is connected • Verify cable connection between AC inlet and power supply • Verify cable connection between power supply and logic board • Verify power supply LED #1 + LED #2 = Power available, and system is powered on. If second LED is not visible when power button is pressed: • Verify power button connection to logic board • Verify power button functionality • Verify cable connection between power supply and logic board • Verify power supply • Verify logic board LED #1 + LED #2 + LED #3 = Power available, system is powered on, and video card found. If third LED is not visible after power on: • Verify that MXM video card is seated properly • Verify logic board If the first LED is not lit the power supply is not working. If the power supply is weak and can't sustain the system, the LED may be lit but unusually blinks or goes out with attempts to power on the machine. This may also happen if you have a bad wall socket. Power up is the hardest thing on any of the components in a computer - RAM, CPU, etc... are pushed to their max and pulling more than operational current. The symptoms you are giving are of a weak, near dead power supply. It may give off all the proper voltages on the pinout, as it is weak - possibly not dead. If you like you can [guide|2740|click here], for a guide that will show you the pinout for what voltages you should see coming off the power supply. The only thing this PSU most likely can't do is pass a current load draw test. You could do that by installing it in another iMac to see if it gives off the same symptoms or you could replace the PSU.