Power Mac G4 won't turn on
If your computer will not turn on there are a couple things to check. First look to see if any of the lights on your computer are on. Also listen for the fan or drive noise. If neither of these devices are functioning, make sure that your computer is plugged into a working power source. You should also check to see that the the breaker on the back of your computer is on the right setting. If your computer still fails to power up then you are probably experiencing one of the following problems.
Dead PMU battery
PMU is the power management unit. If the PMU battery has died then you will need to replace it.
Bad power supply
If you have replaced the PMU battery and the computer still will not start then it is possible you have a failed power supply. It will need to be replaced with a known good unit.
Bad logic board
If this did not work then it is a problem with the logic board which will need to be replaced.
A flashing question mark icon appears on start up
The flashing question mark indicates that your computer cannot locate a bootable operating system.
No startup disk selected
It is normal for the question mark to appear when there is no startup disk selected. This can generally be solved by selecting the proper start up hard disk or installation disk.
Check the keyboard and mouse
If a startup disk is selected and the question mark still appears then check your keyboard and mouse. If a button is being held down on either device then it could cause an error upon start up leading to the question mark.
Corrupted operating system
If neither of the previous steps resolved the issue, it is possible that your installation of the Mac OS has been corrupted and needs to be repaired. Follow the steps given by the CD in order to repair or reinstall the Mac OS.
Failed hard drive
If the installation disk reports that it cannot install to the hard drive then it is most likely suffered a catastrophic failure and will need to be replaced.
Mouse/keyboard do not appear to fuction
The first step is to try a simple restart, as this fixes many issues involved with initializing input devices. If only one input device has failed, replace the non-functioning piece of equipment with a known good device. If replacing the device has no effect or multiple devices failed concurrently, it is likely that the logic board has some sort of hardware problem. Unless there are other available and functioning ports, the only option is to replace the logic board.
No sound coming out of speakers/headphones
Firstly, ensure that all devices are fully plugged in and then restart the computer. After restarting, try checking the audio again If the issue isn't resolved, connect the audio device to a known good system. If the device still does not output audio, the speakers or headphones have failed and need to be either repaired or replaced. If the device functions on a known good system, the audio controller on the Power Mac's logic board has failed and will need to be repaired or placed.
No image showing up on your monitor
The first thing to check is if your computer powers up. Make sure this isn’t the problem by following “Power Mac G4 won’t turn on”. Next check your video cable making sure it is fully inserted on both ends. Then check your monitor power cable making sure it is fully inserted into a working power outlet. Now if you haven’t already, turn on your computer monitor. Most monitors have an LED on the front which lights up green or orange when powered on. If the LED illuminates then try connecting your monitor to a known good computer. If there is still no signal, check your brightness and contrast settings. Turn them up all the way to ensure this is not the problem. If your monitor still doesn't display an image, than you will have to replace your graphics card.