xserve 2006 won't start - how to diagnose?

I recently acquired a used late 2006 xserve, 2 x 2GHz Intel Xeon.

When I connect power cord to it, the power supply fan starts spinning, the green LED on the power supply is on. However, the Xserve does not start. I opened it up and noticed that the 5VSTBY LED on the logic board is ON.

I tried to start the xserve using the front panel power-on button, but nothing happens. On board VR_PWR is OFF. EFI_GD is off as well and I did not noticed it flashing at any point.

I tried the basic troubleshooting procedure outlined in the service manual (Power Supply Verification, Startup), takeing out / moving around memory DIMMs, taking off risers, video card, reseating cables, etc to no avail.

Since I have only one power supply and no other spare parts my diagnostic options are rather limited. So I would very much appreciate any suggestions.