Re: the green led light at the back of the PSU only turns on when I’m pressing the diagnostic button. If the green led is steady and psu fan turns on when you press the bist button, it would suggest psu passes bist test. if possible find another working 24/4 pin psu to double check.  otherwise by dx of exclusion it is probably a motherboard issue.  cpu almost never dies from normal use.
I have a Dell Xps 8700 with 12GBs of ram and a i7-4790k. The gpu is a gtx 470 I added later. The psu is 500 watts. When plugged in, the amber light on the motherboard illuminates (indicating that the 5v rail is working), but whenever I press the power button, nothing happens. The power button never illuminates any color. However, when I press the diagnostic button on the back of the psu, the fans (pc case, gpu, and cpu fans) spin up to full speed, but nothing else happens. When I stop pressing the button, the fans turn off. Someone please help me, I was planning to use this pc as a NAS and a backup pc if my main one breaks.
XPS 8700, won’t turn on