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.
Next look at the diagnostic lights on the logic board and tell us what they do when you try to start.