Abstract: A system and method for diagnosing selective catalytic reduction system of a motor vehicle, the system including an internal combustion engine connected by an exhaust manifold to, successively, an assembly including a nitrogen oxide catalyst and a particle filter and a selective catalytic reduction system, a mechanism draining mass of ammonia stored in the selective catalytic reduction system, a mechanism controlling draining, and a mechanism injecting urea positioned upstream of the selective catalytic reduction system, a mechanism measuring quantity of ammonia at an outlet of the selective catalytic reduction system, a system determining maximum mass of ammonia stored in the selective catalytic reduction system, and a comparison mechanism configured to transmit a fault signal based on a result of a comparison between the maximum mass of ammonia stored in the selective catalytic reduction system and a threshold.