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1.6, 1.8, or 2 GHz G5 processor

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Diagnostic LEDs (4th red)

@mayer or @oldturkey03

Got this system in today.

Heres the issue: When you first turn it on and it works fine. After a second boot (eg, reboot) it fails to boot after the startup chime. Works fine if you leave it unplugged for half an hour and then try.

Heres what happens when you start it up a second time:

  • LED 1 green
  • LED 2 green
  • LED 3 Off
  • LED 4 Red
  • LED 2 turns off

Fans speed up and are cut off by shutdown.

Update (05/06/2018)

Okey Doke. I replaced the Thermal Paste on U22 and on the CPU, it works like a charm!

iMac G5 17" Model A1058 Logic Board Replacement

Hope this helps someone else.

Answered! View the answer I have this problem too

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@captainsnowball here is the meaning of the LED's:

  • LED 1 - Indicates that the trickle voltage from the power supply has been detected by the main logic board. This LED will remain ON whenever the iMac G5 is connected to a working AC power source. 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 source 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 LCD display are communicating. This LED will be ON when the computer is turned on and video signal is being generated.
  • LED 4 - Lights only if the computer detects an over-temperature condition. This LED will be OFF when the computer is turned on and running at the correct temperature.

Try an SMU reset with the switch on the logic board:

The SMU supports the following functions

  • provides initial configuration of system clocks
  • automatically initiates and sequences voltage and frequency slews when the OS requests a transition to a new run level
  • handles all power button and environmental user events
  • controls RPM fans
  • provides processor Mode Ring and thermal data store
  • monitors processor temperature and power consumption
  • monitors VRD10-compliant processor power supply digital interface.

The iMac G5 system employs advanced thermal and wattage management to keep acoustic noise to a minimum. The enclosure is divided into discrete zones, each with independently controlled fans bringing in cool air from the front of the enclosure, directing it over system components and exhausting it out the rear. Temperature and power consumption are monitored by the operating system which communicates with the SMU, which in turn controls and monitors fan operation. If Mac OS X is not booted, thermal management must be provided by the alternate development operating system.


If Mac OS X is not booted and the alternate development operating system does not manage the fans, the fans go into an unmanaged state and run at full speed.

The SMU controls the fans and regulates the speeds to run each fan. The SMU derives fan speed from sensors in each thermal zone. Some of these sensors monitor thermal only, while others monitor both thermal and power.

If the SMU does not receive an update from the operating system within two minutes, it begins to ramp up the speed of the fans to full speed.

Update (05/06/2018)

Here is the location of the SMU button

Block Image

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So why would it be overheating?


Bad fan, bad thermal paste....?????


is the SMU the button?


OK I noticed that is the power button. LED 4 temporarily turns red in a normal boot too.


The top one is the power button which allows you to turn on your computer with the back cover off


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