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No boot, fan spins at full speed


I recently got my old iMac G5 out again but it won't start.

I've checked the diagnostic LEDs. LED 1 lights up when the computer is off after I press the power button LED 2 lights up too and fans start to spin at full speed.

But LED 3 never lights up, I've used my multimeter to check the voltages on the power supply.

The power supply outputs 12v and has two ground connections and a PG (Power Good) Connection which sits around 0.6V.

Can anyone help me. In not entirely sure what the diagnostic LEDs mean nor how I can get my computer to start.

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Are you 100% sure the display is working?


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As you had your system unplugged for awhile I would start off replacing the PRAM battery. Until you have a reliable PRAM voltage your system won’t fully start up. I assume you are getting a boot chime and fan.

Plug the power cord into the iMac G5 and into a known good AC outlet. LED #1 should come on if your computer has been starting up properly.

When the computer starts up, LED #2 will light after you hear the boot chime indicating the logic board is processing power correctly. If LED #2 does not light, go to SMU reset section below for instructions on resetting the SMU. If LED #2 remains off after resetting the SMU, your logic board needs to be replaced. Order and install the logic board for your computer.

LED # 3 will light last indicating that the logic board and LCD assembly have communicated and the LCD is ready to receive video. Does LED #3 come on?

  • Yes: This indicates that your logic board is working correctly to produce video.
  • No: Continue to SMU section to reset system power management.

To reset the SMU - Plug in the power cord while simultaneously pressing and holding the power button on the back of the computer.

After the SMU start up the computer. Does LED #3 light now?

Yes: Press and hold the power button until the computer powers off. Start the computer again and confirm that LED #3 is reliably lighting.

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