2009 Quad Core Mac Pro No Boot, POST fail

I bought a 2009 Mac pro quad core that does not POST. The machine powers up when the button is pushed, all the fans spin on low and the monitor does nothing.
The CPU A/B lights flash when the unit is plugged in as they should.
There is no caps lock light and the machine does not register the keyboard for a PRAM zap. The light on the mouse lights up, but the CD drive does not open when I hold down the mouse button.
I checked the LEDs on the back plane board, the 'stand by' and '12v' light up when the 'test' button is pushed. The EFI done and GPU OK also flash occasionally.
I reset the SMC and RTC.
I did minimum configuration testing. Disconnected everything on the board (HDD, optical, fans, graphics, wireless, bluetooth, front panel, processor board, RTC battery) and jumped the solder pads. No life as it should.
Connected the processor board with no processor or RAM. The CPU error light is on and the power button flashes out the 'No RAM' Code.
Added a CPU and one stick of RAM. It boots as it did before. Fans on low, no chime, no caps lock light.
I checked the CPU plug and replaced the CPU with a W3550 that I found locally. The machine is the same.
Do you know how to isolate the problem to either the processor board or the backplane board?

I figured it out.
It seemed like a good excuse to pick up a dual board. I bought one and popped it in there. Still no boot.

Replaced the backplane board and it's back to life!