Only G5 with older Firmware (til june 2004,1.8GHz) will boot with only 1 CPU (in upper slot) -all later versions needs both CPUs.

@jambalah:
Seems that you have shot one or both CPUs - can be a mechanical crack during remounting. I can only suggest, that these CPUs never (!) should touched, as long as they are running flawless.
Are you sure that all pins (cpu-socket) are ok and not bend? Do you have other CPUs to test?


These early checkings (RAM, etc.) are not done by CPU, but from logic-boards on-board-logic. After all this low-level-tests CPU is initialized at last. If everything is OK and CPUs works, the normal startup-chime will confirm that and system boots to OF.