Official GIGABYTE Forum

Who's the culprit? Gigabyte GA-Z97X-SOC FORCE or Corsair AX1200i PSU?

So, I am back from my summer holidays.
Pc has been off and unplugged from the mains for over 20 days.
Two days ago I felt like playing a little: Elite Dangerous or Far Cry 4.
I sat down at my desk, dusted off my display, mouse and joystick...switched PC on...switched PC...o...hey, wait a minute?
PC was dead. I unplugged the cable, switched off the PSU and re-plugged it several times. Nada.

I run the self-test with all the cables attached (it is a modular PSU), it won't even start. With all cables detached (as per manual) the self-test runs and is successful (green led).

After several attempts I found out that when the small part (6x2 pins) of the cable running form the PSU to the MB is attached, the test will fail. When only the bigger part (7x2 pins) is attached, the test is successful and the mainboard and system fans are powered. (obviously PC won't start as there is still lack of power).

What do you reckon? Before proceeding with an RMA, I would like to understand a bit better which component is more likely to be faulty: is it the PSU or the MB? Or maybe just the cable (very unlikely, IMHO)?

Unfortunately I don't have other PSUs not MBs I can try.

Any suggestions?
CASE Corsair 600T, PSU Corsair AX1200i, MoBo Gigabyte GA-Z97X-SOC FORCE, RAM Corsair Dominator Platinum DDR3-2133 16GB (2 x 8GB), CPU i7-4790K, CPU COOLER Noctua NH-D1, GPU MSI GeForce GTX 970 4GB Twin Frozr V, SSD Samsung 850 Pro Series 256GB