Method for monitoring an SCR catalytic converter

A computer program product having program code for execution executed on a computer or a control unit (19), the program code stored on a non-transient machine-readable carrier for carrying out a method of monitoring an SCR catalytic converter (12), in which method the NH3 storage capacity of the SCR catalytic converter (12) is monitored, with the SCR catalytic converter (12) initially being filled in an overdosing phase with a superstoichiometric reducing agent dosing up to the maximum NH3 storage capacity, subsequently being acted on in an underdosing phase with a reducing agent dosing which is reduced or eliminated in relation to a normal dosing, and with the NH3 storage capacity being indirectly determined during the underdosing phase by determining at least one characteristic value which is dependent on the NOx conversion rate, characterized by

• continuously measuring a variable which is characteristic of the NOx conversion during the overdosing phase, and
• inferring that NH3 slippage (25) is present, indicating a transition from the overdosing phase to the underdosing phase, when the NOx conversion falls.