A method for operating an exhaust purification system of an internal combustion engine

The invention relates to a method for operating an exhaust purification system (1) for purifying an exhaust gas of an internal combustion engine which can be operated in a lean operating mode with excess air and in a rich operating mode with a lack of air, wherein the exhaust gas purification system (1) in a flow direction of the exhaust gas arranged in succession - an ammonia formation catalyst (5), capable of catalyzing under reducing conditions, an at least partial reduction of nitrogen oxide contained in exhaust gas to form ammonia, - a first exhaust gas sensor (S1), - an ammonia-SCR catalyst (7) with a storage capacity for ammonia, - an ammonia-SCR catalyst (7) immediately downstream nitrogen oxide storage catalytic converter (8) with a storage capacity for oxygen and a storage capacity for nitrogen oxides, and - a second exhaust gas sensor (S2) having. In this case (S1, S2) are used exhaust gas sensors as first and second exhaust gas sensor, which (each a correlating with a nitrogen oxide content of exhaust gas first signal (S In a diagnostic operation in immediately adjoining operating phases of the internal combustion engine with the lean operation mode and rich operation mode by evaluating the signals of the first and second exhaust gas sensors (S1, S2) the ammonia storage capacity of the ammonia-SCR catalyst (7), the oxygen storage ability the nitrogen oxide storage catalytic converter (8) and optionally to determine the nitrogen oxide storage capacity of the nitrogen oxide storage catalytic converter (8).