MacPro with blinking front light, and boots to grey screen

What does it mean when the front light blinks.
It's constant signal blinks.

Trying to trouble shoot a computer and not getting anywhere.
So far I have swapped out two known good boards, ram boars, video cards, processors, HD/power cable (cable for all 4 bays). Now I'm getting this blinking light.

Right now the computer starts up no problem but just get a grey screen. It dongs as usual, and will take the PRAM reset keys but will not go into FW mode.

Any ideas.

Thanks,