No image when inserting GPU in working rig

[lazarba]

So I built a computer for a friend of mine some days ago . I got him :
i5 6600K with a CM EVO, not OCed yet
Gigabyte K3 G1 Gaming z170
Corsair Vengeance 8gb single stick and a
Superflower Golden Green 750W 80plus Gold .
so we set the whole thing up ( inside a Corsair Spec 2 and along with an ocz trion 150 240gb) and everything is perfect. Its working great with the onboard HD530 GPU. ALls good.
Now , we had decided that I would sell him my R9 390 once the GTX 1070 arrived in Europe so I could get it, but I also had an old HD6850 lying around so decided to give it to him until I sold him the 390. I go over to his place , sit the GPU in the first PCI slot, connect the 6pin connector and the HDMI cable, I turn the pc on and... Nothing. Well , nothing appears on screen at least. The GPU fan though, thats running at a 100%. I changed the connecting cable, tried different slots in the PSU, tried different slots on the motherboard , NOTHING does it . I really am at loss here, because this GPU is alive and well. I actually just tested it back on my very own rig ( along with an i7 2600, EVGA Supernova P2 1000W , 8GB XMS3 and an Asrock H61M-DG3/USB3 ).
To sum up :
we have a perfectly working PC and a perfectly working GPU, but once we plug those two together the GPU fan spins at 100% and we get no image.
Could it be the PSU? The motherboard? Well the PSU works fine without the GPU and its not like its a power hungry card. On the other hand, both PCI slots being faulty is way too weird, statistically speaking, especially since the motherboard shows no other signs of error.
Help guys!

PS.
I also checked the BIOS a bit, its set like this :" Primary display card : PCI slot 1".
Regarding the onboard gpu , its set to :"Auto".

 

[Themastererr]

The PCI-E lanes are on the CPU in a 6600k build, doubt the slots would be dead, the onboard controller on the CPU might be though.

Best bet is to do a full CMOS reset, then take out each stick of RAM but leave the one closest to the CPU in, then try to boot. Once it's booted turn it off and put the remaining RAM in.

 

[lazarba]

Actually I did reset the CMOS and tested the ram on all 4 slots. I am uncertain if I did those 2 simultaneously though

 

[Themastererr]

Ok, you are plugging the display into the GPU when trying to boot from it correct? Yes, I've seen people that think it still comes out of the motherboard.

Have you tried wiping down the PCI gold fingers? How about changing the power connections to a different cable?

Check all other connections as well, CPU power, ATX power, chances are you could have knocked something loose when installing the card.

We know the GPU works in another build, it's not dead. Keep tinkering with it. You can also try taking apart everything and booting it on a table to rule out any shorts.