0d error

It only works using channel A, that means there is a good chance channel B on your
memory controller is either completely dead or can't handle your particular RAM
kit (in which case it is defective).

As I said before the IMC on the CPU has 2 channels, A and B. If one is working then
using that channel would result in a stable and fully usable system.

You can try checking your CPU for any missing or bent over pins, that can also
result in a channel dropping out but it's unlikely. The odds of a single pin
being perfectly bent in such a way that the CPU still fits in the socket is low.

The way the system RAM operates faster is by using 2 channels to the CPU. 2 channels
means double the bandwidth to and from the CPU and RAM. A single channel working
means the system still works but the performance takes a hit. Dual Channel vs
Single Channel performance. The system is behaving as if you only have 1 stick
of RAM installed because both are being accessed through a single channel rather
than split between 2. Basically the connection between the CPU and any RAM you
install in channel B is cut and given it's broken inside the CPU itself there is
no way to fix it. The CPU can still see and use RAM in channel A however so the
system continues to function normally given the RAM channels function independently
of one another. Like pulling a spark plug lead off a 2 cylinder engine while it's
running, you lose some power but the engine will continue to run. In the case of
a CPU however it won't damage the system to do so, you just lose half your peak
memory bandwidth.

If the CPU is new I would return it, simply because it isn't functioning
correctly, even if it is stable. If it is out of warranty there is no problem
using it in single channel, all you sacrifice is some RAM performance and in
most use cases you wouldn't notice the difference anyway.

I hope that makes sense.