Maybe disable some CPUs?Power Mac G5 has started having this weird issue where it shuts itself off during startup (always at the part where the Apple logo comes on screen). Have tried PRAM reset, NVRAM reset, PMU reset, swapping drives, swapping GPUs, blowing dust out of the case, vacuuming dust out of the case…nothing fixes it. PMU reset at least gets me to the desktop, but the Mac will inevitably shut itself off again. Debating whether to replace the power supply or just get a new G5…
I use the following command in Open Firmware for my Quad G5 to remove the last CPU:
setenv boot-args cpus=3
You may want to try cpus=1.
But the failure for my Quad G5 appears as a hang with the fans turning on full speed instead of a shutdown.