If there’s no video output, what can be the reason
It starts up, the chime sound is there, the fan spins but no video?
It really can be multiple possibilities, best to take it to repair shop or apple for diagnostic than fix it your self and change that part but here is a few thing that can be the reason, bad gpu.. maybe as amd graphic card do have issues in mac pro aka trash can, try starting us using safe mode,
Booting up safe mode / Restart your Mac, then immediately press and hold the Shift key. Release the Shift key when you see the login window.
If you’re able to boot safe mode than you know you have Gpu issue and need to replace one or both gpu cards.
New, if safe mode is not helping, than make sure your monitor is oak, your osx is ok, and finally the actual video port your using is still giving signal..
if your using Hdmi for video, try using display port ( thunderbolt ) just to make sure issue is not the port…
We have a Teardown Video for this model, you can watch to see all the parts inside.