Its probably the cable running down the right hinge that has become damaged. If you look up into the hinge you should see a black cable. Does it look damaged?
[CLOSED] Left side doesn't work except in mono
My warranty recently expired. And last week my headphones stopped working on the left side however both cans work in mono. I tried switch the cable and there was no improvement. I believe the left channel is totally not getting into the headphones, and I am only receiving right channel signal...which I hear in both cans in mono.
I just took closeup pics of both hinges in the link below. There are no visible tears in the exposed wires, its possible its deeper.
I highly doubt its hinge issue, because it seems like both cans can receive proper signal when switched to mono. It's more like, its completely not receiving a left signal at all.
Have you tried different sources? If there is a screw-on adapter, have you made sure it is connected tightly?
Dude the same thing happened to me exacly one week after the warranty expired, what are the odds, i really think pioneer makes it on purpose..