Apparatus, system, and method for estimating an NOx conversion efficiency of a selective catalytic reduction catalyst

x x x x 3 3 x x x 3 According to one exemplary embodiment, an apparatus is disclosed for estimating an NOconversion efficiency of an SCR catalyst. The apparatus includes a catalyst degradation module for determining an SCR catalyst degradation factor and a NOconcentration module for determining an SCR catalyst inlet NOconcentration based on an interpretation of at least one NOdetector signal. Additionally, the apparatus includes an NHconcentration module for determining an SCR catalyst inlet NHconcentration, a temperature module for determining an SCR catalyst bed temperature of the at least one SCR catalyst, and a space velocity (SV) module for determining an exhaust gas SV for the SCR catalyst. A NOconversion efficiency module calculates a NOconversion efficiency value based at least partially on the SCR catalyst degradation factor, the SCR catalyst inlet NOconcentration, the SCR catalyst inlet NHconcentration, the exhaust gas SV, and the SCR catalyst bed temperature.