If you have an HD that you know is good, it could be the cable. If you have an external case for the HD (Firewire only) you could try to boot from it, or a Target booted Mac (again via Firewire - won't USB boot),
Have you seen this [[Power Mac G4 Quicksilver Troubleshooting|diagnostic]] guide… is there a display attached… some of these would not run headless.
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