A method of detecting ammonia in the exhaust system includes detecting a predetermined engine operating condition. Upon detecting the predetermined operating condition, the method determines a first NOx conversion efficiency of a catalyst at a first time T1. The method then injects a reactant into the exhaust upstream of the catalyst and determines a second NOx conversion efficiency at a second time T2. The method then processes the first and second NOx conversion efficiencies to determine whether an ammonia slip condition exists.