Refurbs have very short warranties. Exercise yours ASAP. They should have a battery of tests before they issue RMA
If you want to dig deeper, pull out manual and "rebuild" based on instruction, but stop and test before you put in RAM and video card. You "should" get an error for one or the other, then 'fix' that and you should get 'error' the next one which you can then 'fix'.
I usually mess up with a poorly connected power cable...24 pin, 4 pin, 6 pin OR a poorly connected CPU fan.