Abstract: A method for operating an SCR catalytic converter in an exhaust gas system of an internal combustion engine with ammonia dosing upstream of the catalytic converter. The method includes: determining, on the basis of a catalytic converter model, the efficiency of nitrogen oxide conversion in the catalytic converter; determining an ammonia fill level in the catalytic converter; determining a nominal ammonia fill level in the catalytic converter, based on the determined efficiency and a pre-definable target nitrogen oxide conversion; and controlling the ammonia dosing depending on the nominal ammonia fill level and the ammonia fill level.

Abstract: In order to determine the conversion efficiency of at least one component of a catalytic converter box comprising at least one SCR catalytic converter and an oxides of nitrogen storage catalytic converter for the aftertreatment of the exhaust gases of an internal combustion engine, wherein the catalytic converter box is associated at the downstream end with a NOx sensor, at least for a limited period of time the internal combustion engine is operated such that a first of the two components does not affect the signal detected by means of the NOx sensor and the conversion efficiency of the second component is determined depending on the signal of the NOx sensor that is not affected by the first component.