Motherboard and CPU combination not booting

[nexere]

Hello, i am building a PC for my brother.

I have bought a Gigabyte B360M DS3H motherboard and an i5 8400 my personal pc is a Gigabyte Z370P D3 and 8700k.

The B360M DS3H doesn't seam to be working with the i5 8400 cpu, it lights up for around 3 seconds and then shut off and reattemps booting a second later.

I have testing both these parts by swapping them with my own PC, a X370P and 8700k and it always works, only the combination of the i5 8400 and B360M DS3H won't work.

i have already replaced the motherboard once believing it was the cause before pulling my own PC apart.

after buying a speak it beeps when boot without memory (which i have already swapped with my PCs) but no beeps without.

Its bread-boarded to rule out any external shorts and i have also swapped the PSU.

Any one got any ideas on what this could be? all i can think is to buy and use a different motherboard model?

 

[CountMike]

What about BIOS version(s) ?

 

[nexere]

What about BIOS version(s) ?

Hello Mike, it was running F02 which is the release version, which should support this CPU,

https://www.gigabyte.com/Motherboard/B360M-DS3H-rev-10#support-cpu

Though the new bios says it updates the CPU microcode.

"Update CPU ID/Micro codes in new BIOS means?
[1] To support new series of CPUs, or [2] To support the same CPU but with new stepping. CPU support of motherboard is consist of BIOS and hardware design, detail CPU support information please see CPU support list."

Surely there couldn't be a stepping change that stops the i5 8400 from working without a bios update? (i updated the bios on my own PCs motherboard which is a 370 but also Gigabyte)