Exhaust gas catalytic converter system and method for operating an exhaust gas catalytic converter

A method is disclosed for operating an exhaust gas catalytic converter designed to reduce nitrogen oxides in an exhaust gas flow using a solution that contains urea and that reacts to form at least ammonia in the exhaust gas flow. The method may include: introducing an amount of the solution containing urea upstream of the exhaust gas catalytic converter until an ammonia slip through the exhaust gas catalytic converter; determining a first actual value of the loading state of the catalytic converter with ammonia at the time of the slip; determining a first estimated value of the loading state at the time of the slip by means of a dynamic model for the loading; subsequently draining the ammonia from the catalytic converter; determining a second actual value of the loading state of the exhaust gas catalytic converter with the solution containing urea when the catalytic converter is drained as specified.