Abstract: An exemplary embodiment is a system, including an internal combustion engine fluidly coupled to an exhaust gas flowpath, an aftertreatment system disposed in the exhaust gas flowpath, where the aftertreatment system includes a NOx reduction catalyst. The exemplary system includes a processing subsystem having a controller, where the controller includes modules structured to functionally execute operations for determining a catalyst degradation. The modules include a test conditions module, a testing module, a monitoring module, and a catalyst aging module.