how to do firmware reset?

hi
i am using a G4 500 MHz power mac. there is some problem in my mac i.e the PPC doesnt boot at all. i reckon there is some problem in my firmware ? so i tried reseetting it using:
1. command+opt+p+r
2. Hard reset
3. reset-NVRAM also.
is there anything more i can do to reset the firmware.
-Chanakya

How do you know that is a firmware problem?

When you say that your computer does not boot, what do you see on the screen?