Official comment
@Tom > Sounds like the channel fader may be faulty or it's assigned to the crossfader (and it's to the wrong side), but that seems less likely if you've tested a few things.
The CH2 issue could be on the input gain stage - best way to troubleshoot that is swap the input L/R cables and if the problem stays on the same side, it's within the channel itself.
You can contact Pioneer DJ technical support in your region to have your hardware inspected and/or serviced.