G4 Auminum1.67. originally I had the ability to port out to another monitor but that stopped working. Because the symptoms were classic Inverter Board Failure, I secured a "new" not used Inverter Board and installed it.
(a.multi colored lines on screen, then image would lock in and be good.
b. LCD stayed black during boot process.
c. Attached to external monitor and had an image. The next reboot with out monitor produced a normal LCD image.
d. Next boot up resulted in black screen again. e. The external monitor had no image as well.Stopped

Now, after replacing the Inverter Board, The screen flashes/flickers white either at the top or bottom on boot but then remains black during boot up. And finally, the machine shuts down. (There is a boot chime and the hard drive and fans spin up.)

I tried booting to another monitor again and no output to the external monitor either. If a problem like this isn't an inverter, What else could it be?

Tried PRAM, NVRAM rests with no success.

I've searched several forums and I have attempted most of the options listed. I hate to start buying parts on speculation.

typically that would be the inverter cable. usually it slowly gets damaged in the hinges and then the bare wire can start shorting out the backlight. but if the external monitor is no longer working, it may be the logic board.