Question Can't get RX570 to work

[Slov01]

I bought a Gigabyte RX570 4GB and new power supply (VS550) for my aged computer, but when I install it, it beeps 8 times and won't post.

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Motherboard H61MLV2 (1 PCIe x16 slot)
8 GB RAM
Pentium G645 2.90 GHz
Various SSDs
Win10
Old GPU: MSI HD 6670 2GB
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Things I've tried:

• Updating motherboard BIOS
• Clearing CMOS
• Using the minimum numbers of RAM and SSDs to boot
• Swapping old GPU back in (still works)
• Checking new GPU is inserted correctly
• Disabling/re-enabling iGFX
• Changing some uefi/legacy motherboard settings
• Resetting CMOS again
• Power LED on GPU indicates power supply is sufficient

I know the motherboard should work with the GPU as there are builds out there. Not sure CPU will utilise PCIe 3.0 but that is another problem.

The only thing I can't do, is test the GPU in another computer. Is there something else I can do which will rule out a dead GPU?

 

[Slov01]

I've checked the GPU in another computer; it works fine.

Would the CPU (Pentium G645) be causing the problem?