I was given a dysfunctional MDD G4 PowerMac recently...this is the dual 1.42 FW800 model. The battery voltage was low, so I replaced the battery. It started up fine, but on the next attempt, it refused to start when I pressed the power button. I tried (sequentially) resetting the PMU, PRAM, NVRAM, removing the battery, unplugging from AC. The only way I can get it to start up reliably is to unplug it for a minute, after which it powers on just fine.
I don't know if it's worth the trouble of a Genius Bar diagnosis, but will consider any ideas. Thanks.