Mac Pro 2008 no video screen after changing the graphics card

Hello everyone,

I have a 2008 Mac Pro 3,1 with Nvidia Geforce 8400 GS 512 MB running on Mac-OS v10,11 el-capitan. It's a pc graphics card with no boot screen. Previous owners had already changed its native Radeon 2600 XT card because of an upgrade (in their own mind!!)

Now hence i want the boot screen back, i have bought a Radeon 2600 XT (it's a pc version too, and is 512 MB, not the Apple's native 256 MB). I know it needs to be flashed to have the boot screen... But there's another problem, when i change the graphics card to 2600XT, i lose the screen!

I hear the chime sound, and the computer status is normal, but there's no screen.

I also tested with other graphics cards too. I have a Radeon 5870 (1 GB, pc version, not mac native) and a Radeon 5970 (2 GB, black edition by XFX).

With 5870 which is unflashed, the screen gets back and is OK, but the same problem happens with 5970, i mean the screen remains black. I will also need to use my 5970 on future too. They both need to be flashed.

I have red several topics here, and found that some people have the same problem, but their machine was Mac Pro 5,1 not like mine, the 3,1

Well their solutions didn't work for me too. I tried the NVRAM reset, SMC reset; but it didn't help.

Long story short, i think the problem is not with the cards being unflashed .... The problem is I don't have screen with some of my graphics cards (Radeon 2600 XT and Radeon 5970) ... But some others work properly (Radeon 5870 and Nvidia 8400)

So, can you please help me solve this problem? I'm really confused! Please help me.

Thank you 🙏🏼❤️