iMac G5 powers on but no display

I seem to be getting iMac G5 donations left and right these days...the latest one I've received powers on but nothing comes up on the screen. Looking at the diagnostic LEDs, only 1 and 2 come on which indicates good power and I understand #3 has to do with the display but I'm not sure if this means screen, cable or possible GPU failure. Would love to hear about anyone else's experience with this problem and what the most likely problem is.

What happens if you plug in an external display?

- Nick