XPS 8940 2 Whites and 4 Oranges Lights.

XPS 8940 blink code 4 amber, 2 white = CPU Power cable connection issue 

For starters, run the PSU BIST using the instructions for a PC without a PSU test button.

Assuming PSU passes the BIST, check the CPU cable connections to the motherboard, both #2 and #4 in the motherboard picture.  Carefully unplug each one and then reconnect it to the same motherboard connector.

Might also be a good time to install a fresh CR2032: 

1. Power off, unplug power cord from rear of PC
2. Press/hold power button for ~15 sec
3. Open case and remove motherboard battery (check Service Manual for details)
4. Press/hold power button for ~30 sec
5. Install a fresh CR2032 3-volt coin cell battery, ~$3)
6. Close up and connect mouse, monitor and keyboard
7. Reboot