G5 wakes up for no apparent reason I’ve got a very odd problem with an old G5 of mine. It's a 2.3GHz Dual-Core G5 with Mac OS X 10.5.8. Sometimes, though not always (about 50/50), when putting the computer to sleep, it wakes up after a couple of seconds. This is really puzzling and I just can’t figure out what the problem is. I've had this computer for many years now, and it started just recently, but I can't pinpoint the exact time.

So far, I can rule out the following potential causes:


Wake for ethernet network administrator access (turned on or off, doesn’t matter)
Any network traffic - the problem can occur when the G5 is not connected to the network
External USB hubs (none connected)
Bluetooth (doesn’t have BT)
Modem (doesn’t have one)
Software (clean installation of OS X: same problem)
Keyboard or Mouse waking it up - the problem can even occur when none are connected and I put it to sleep via SSH
Graphics card/monitor: problem can occur when there is no monitor connected and I put the G5 to sleep via SSH.
Power fluctuations


The logs don’t help me. The Wake event is always 0020, regardless whether it's an intentional wake or not.

When the G5 woke up, I can put it back to sleep and then it will stay in sleep mode. So it’s no big deal but I still would love to know what the problem is.

Edit (21-June-2012): Thank you all for your suggestions and pointers. Unfortunately my G5 recently had a failing power supply, and it won't wake up from that anymore.
Have you tried Reseting the SMU


  The SMU (System Management Unit) is a microcontroller chip on the logic board that controls all power functions for your computer. If your computer is experiencing any issues regarding these functions, resetting the SMU may resolve the issue. The functions controlled by the SMU include:
  
  
  It tells the computer when to turn on, turn off, sleep, wake, idle, and so on.
  It handles system resets from various commands.
  It controls the fan.
  
  
  Note that resetting the SMU does not reset the PRAM.




Steps to reset the SMU on:


  Power Mac G5 (Late 2005)
  
  To reset the SMU on a Power Mac G5 (Late 2005) computer, either use the steps listed above for the Power Mac G5 (Late 2004) or:
  
  
  Turn off the computer by selecting Shut Down from the Apple menu or
  by holding the power button until the computer turns off.
  Open and remove both the metallic outer door and the inner plastic
  air deflector.
  Remove the fan assembly immediately to the left of the processor
  module.
  Press the SMU reset button on the logic board.
  Replace the fan assembly, air deflector, and outer door.
  Turn on the computer.
  
  
  The SMU reset button is located underneath the lower bank of system memory slots, as shown below:
  
  
  
  Unlike earlier models of Power Mac G5 or Power Mac G4 computers, the Power Mac G5 (Late 2004) and (Late 2005) models do not have a PMU. This functionality has been replaced by the SMU.