why doesn't my mac fully work?

Open it up and check over the capacitors. These early G5s were prone to failure due to an industrial espionage incident in the Orient, where a stolen incomplete formula for the capacitor juice was used. (Not just Macs were affected.)

Follow the instructions here to get the back cover off: iMac G5 17" Model A1058 Power Supply Replacement

Carefully examine all the caps for swollen tops or even brown foamy stuff leaking out. Shine a flashlight into the holes of the power supply; you can see one or two caps if you look very carefully, and sometimes you can see if these are affected.

If you have bad caps, there are shops that specialize in replacing just the caps for a lot less $ than a new logic board. Google to find a repairer.