Question New PC build wont POST any longer after unplugging an RGB header while it was on


Oct 13, 2010
I got a new case, cpu, mobo, and AIO cooler recently and slapped them all together. It worked first try no problem, and I used it for two full days without a single issue, I was very happy with it.

I was inspecting the RGB controller for my cooler master ml360r AIO and noticed there was a redundant cable that I didn't need for my purposes. It was plugged into the "MB 12V" slot on the cooler masters RGB controller. And plugged into my motherboard's addressable LED strip header, which my manual says has a maximum power rating of 5V. I didn't need this, but I had it plugged in because I followed some guide where they used it for their needs, but I don't need it. If it matters, it was plugged in correctly though, ensuring the power pin was on pin 1 like my mobo manual says.

So um.... I unplugged it from the motherboard while it was... on. I had assumed it didn't carry power, I thought it was just a signal, a way for the motherboard to control the lights via the controller or something like this? I realize this seems silly now in retrospect, as it literally says "12V" on it. But I am new to RGBs in general, much less RGB controllers.

Anyways, my PC immediately shutdown when I unplugged this from the mobo. And when trying to boot it back up it would come on for a few seconds and them shut down. And it was stuck in a boot loop over and over, never reaching POST. The DRAM error light is lit up, and I notice the CPU error light will flicker on and off but it doesn't stay solid like the DRAM light.

So I turned it off and did some tests. I have another mobo and cpu in another case. I can do some form of testing. I moved my RAM over and plugged that board in. And the PSU checks out, as I can POST on that board with it. But after testing every single stick of RAM one at a time, I found out one of the four sticks is dead now. I can post with three of the sticks, one stick makes it shutdown itself down before POST. So I guess that one died when I... idk shorted out something?
Is "short out" the right phrase here? Whenever I unplugged that RGB header obviously something happened. But there was never any smoke or fire or any burning smells, not even a weird pop or any other strange noise. It just shut off.

So I have a working PSU and three working RAM sticks (and my GPU is ok too). The only things I cant test are the CPU and the mobo. So I reached out to amazon for a replacement and they sent me a new board (exact same model) next day shipping. I took everything apart and put it all on this new board. Sure enough, its behaving EXACTLY the same as the other board. Even with just one CONFIRMED working stick of RAM, the DRAM error light is on. And the CPU light keeps flickering on and off.
Also, to clarify, when I have confirmed working RAM in the machine, and turn it on, everything comes on as normal. Every fan, including CPU fans. And it doesn't shut off, it just stays on forever. Never POSTing, but also never turning off. Does that happen when the CPU is the only dead part? I know dead RAM makes my PC just shut itself off. But what should happen if the CPU is indeed the point of failure?

So the only thing left to test is the CPU now right? Unless I received a dead on arrival replacement mobo? Or can slotting in a dead RAM or dead CPU also make the brand new mobo die too? God I hope not.

Anyways, Im also getting a free replacement CPU (exact same model) but it wont be here for a bit. So Im just wondering if this sounds like its indeed the CPU at this point. I confirmed that one of the RAM sticks did in fact die when I unplugged that RGB header, but can a CPU die the same way? At the same time? Any help would be greatly appreciated.