well a few days ago i changed my motherbord to an asusz77-v pro/thunderbolt. when i had put everything in its place in the computer i didnt get any screen. so i took the things out one by one and it seemed that the gpu was the problem but i didnt get the VGA_LEd on the mobo that should lighten up if there's an error with the card. then i tested without the gpu and it worked the system boot and everything worked. so i tested my old gpu but no screen this time to and then i tryed in the second pci-e slot i didn't work this time to so i have no idea what the problem is the gpu fan are spinning when i start the computer and i 1200watt in and dont ask if i have forget the 6pin becuase a red light is displayed on the gpu. what could be the problem please help and sorry for my bad english.
Can you test your VGA in other system? If they are/were working then your motherboard is messed up for sure. It looks like your new motherboard may have some issues with PCIex slots ... what are the chances that both your VGA turned bad? Anyway, try to load default bios settings. save it and restart. then put in your VGA.
make sure the 4/8 pin pci power plug on the mb is plugged in and locked. make sure the mb bios has all of it updates. in the bios with the new cpu the ipgpu is set as the main video output.change it to pci/peg.on the new asus mb with ivy cpu you can change the pci bus sped from pci 3.0 ant 16x to pci 2x by 16. on the main video card slot. with your older video card try setting the pci bus to 2.0 mode.