To fully reset a Power Mac G5, you need to reset the SMU and the PRAM. The PRAM (parameter random access memory) is where information relating to many system settings is stored. If the problem relates to any system settings, serial port configurations, disk cache, monitor depth or the RAM disk, try resetting the PRAM. The SMU (system management unit) is the chip that controls all of the power functions on the computer. If the computer is not starting, not displaying video or not going to sleep, the problem will probably be resolved by resetting the SMU.
Resetting the PRAM
Turn the computer off by selecting "Shut Down" from the Apple menu.
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Turn the computer back on via the "Power" switch.
Hold down the "Command" key (the key near the space bar with an Apple pictured on it) while simultaneously holding down the "Option," "P" and "R" keys. Do this immediately upon turning the power back on.
Keep holding the keys down until the startup sound is made twice.
Resetting the SMU, Late 2004 Model
Shut down the computer by selecting this option from the Apple menu.
Disconnect the power cable from the machine and wait two minutes.
Plug the power cable back in, turn the machine back on.
Resetting the SMU, Late 2005 Model
Turn off the computer and remove the power cable.
Remove the metal side panel. To do this, locate the rectangular metal latch on the back of the computer, toward the top and far left. Pull this switch up to disconnect the panel. Lift up and pull out on the side panel.
Remove the plastic shield. There is a clear plastic shield protecting the insides of the computer, it has a handle at the top and the middle. Grasp the handle, pull out toward yourself and up to remove.
Remove the metal shield. This is the large metal piece with "G5" printed on it. Slide it to the left to unlock, pull the top part toward you to loosen it and then pull the shield out vertically.
Remove the fan. The fan is the large plastic piece to the right of the metal shield. Pull this straight out toward you. Removing the fan reveals another plastic piece, which is the air flow vent, this should be removed by pulling directly out toward you.
Locate the SMU reset button. This is located behind where the fan was. You see three rows of black memory slots. Look under these and on the right side, you see a small metallic button. Press this button.
Reinstall the air flow vent, fan, metal cover, plastic cover and metal door. Restart the computer.
If the computer is simply not responding, turn the computer off by holding down the "Power" button on the outside of the machine. Turn the power back on but do not reset the SMU or PRAM. If there are still problems, reset the PRAM. If the problem persists, reset the SMU.