Method and apparatus for evaluating an ammonia conversion efficiency of an ammonia oxidation (AMOX) catalyst for reducing ammonia and hydrocarbons present in an exhaust gas stream. The AMOX associated with an exhaust gas after-treatment system has a selective catalytic reduction (SCR) catalyst for reducing nitrogen oxides (NOX) present in the exhaust gas stream and a dosing system for introducing a reductant into the exhaust gas stream for reacting with the nitrogen oxides in the presence of the SCR catalyst. The AMOX catalyst is arranged downstream of the SCR catalyst.