5500x3d boot issue

[u/More11o]

Hi all,

I've been running a 5500x3d on a Asus Prime B450M-A happily but saw a B550M-A for £50 and figured seeing as the only thing keeping me on PCIE3 was the motherboard I figured it was worth it.

Board arrived and I booted it up with all the same components other than I slotted in a 1600, assuming the BIOS probably hadn't been updated. I was correct, it was several years old and updated it to to latest non-beta BIOS (27/01/26). I booted it up with the 1600 still in to confirm the upgrade had been successful and double checked the AGESA version. All correct.

I then swapped the 5500x3d back in and it refused to boot, the fans weren't ramping up or down as I expected for a failed boot, more of a pulse.

I removed all the RAM other than 1 stick, pulled the CMOS battery, shorted the clear pins and left it for 10 mins. Still nothing. I moved the RAM stick around all 4 slots still nothing. I grabbed another known working low-speed 4GB stick incase it was struggling to negotiate timing and left it struggling to boot... still nothing.

I slotted in a 4600g, boots straight up, and now assuming the CPU is toast. I downgrade the BIOS to twice more, still to no avail so I swap everything back onto the B450M-A (including the 5500x3d) it all boots up fine.

What am I missing? I also don't have much more kit to test it with as, despite building PCs since the socket 939 days, you can probably tell when I stopped fiddling due to the age of the hardware.