What does P2BAC code on 2011 Ram 2500 diesel mean?

EGR Deactivated ( Plugged? ) NOX exceeds allowable spec.

NOX are Oxides of Nitrogen and nasty smog causing junk.

Common for this year to plug the EGR cooler or have an EGR failure.

This is also one of the codes you run into if you don't do your emissions delete correctly.