Dell Precision T3600 PSU Cable Failure

I have a Dell Precision T3600 series (2012-2013) that will not go to BIOS. Instead, it gives me the following diagnostic LED signal: Amber Power LED, Amber Blinking 1, Amber Blinking 3, Amber Blinking 4. (Only LED 2 is off completely.)

I researched and found this LED configuration to mean that the computer is recieving power, but the PSU cable might not be connected to the motherboard. I checked the connections and they are solid.

Does this indicate a problem with the PSU to Motherboard cables, or a problem with the motherboard itself? I do not know what the main power rail is but I am researching.

If it is the latter, is there any way I can localize the issue on the motherboard by reviewing the motherboard layout/schematics, so I won't need to purchase an entire motherboard replacement?

I have attemped various configurations with my system, including but not limited to: different power outlets; changing the CMOS battery, and attempting to run it with the CMOS battery out; trying to run the computer without the CPU power cable connected; attempting to run it without the GPU; moving the PSWD jumper plug to different places like the PWR_REMOTE connector, RTCRST connector, and an unammed 2 pin connector next to the main power supply connector.

All failed to make any difference, except for:
1: Connecting the PSWD jumper plug to the PWR_REMOTE connector turned all diagnostic lights and fans off after about 3 seconds
2: Connecting the PSWD jumper plug to the RTCRST turned all diagnostic lights and fans off instantly
3: Disconnecting the CPU power plug caused the lights on the GPU to turn on, but the diagnostic lights gave the same symbol.

I also tried, just for good measure, the steps listed here:

No dice.

Ideally I would not like to order any replacement parts except for small components. I feel like throwing out the entire motheboard is a waste.

Thank you for any help you are able to provide.

EDIT: I just learned what rails are. Is it possible/safe for me to check the voltages on each pin on the cable from the PSU with a multimeter? To ensure each one is outputting the right current? Or could I check the voltages on different parts of the motherboard? (I would only be able to if schematics/layouts were available, and if the problem is on the top or bottom layers)

And again, thank you.