A method for operating an emission control system in an exhaust-system branch for an internal combustion engine, wherein the emission control system has at least the following components in the direction of flow of an exhaust of the internal combustion engine: an NO x sensor which emits an NO x signal, an introduction device for introducing a reducing agent into the exhaust-system branch, a temperature sensor which emits a temperature signal, an SCR catalyst (selective catalytic reduction) and an NH 3 sensor which emits an NH 3 signal and a second SCR catalyst and/or an ASC catalyst (ammonia slip catalyst), and also a separate control unit for evaluating and/or controlling at least one NO x sensor signal, a reducing-agent sensor signal and an exhaust-gas temperature sensor signal, wherein the control unit has a filling-level model of the SCR catalyst, characterised by the following method steps, which are carried out at regular intervals: - measuring the exhaust temperature; - upon exceeding a defined temperature and once a defined NO x mass has flowed through the SCR catalyst, by deliberate, brief exceeding of the slip boundary of the SCR catalyst, abruptly increasing an introduction amount of reducing agent in the exhaust-system branch until the NH 3 sensor detects NH 3 slip; - correcting the filling-level model by overwriting the currently calculated NH 3 filling level of the SCR catalyst by the maximum filling level at the current temperature; - reducing the introduction amount of reducing agent. an NO x sensor which emits an NO x signal, an introduction device for introducing a reducing agent into the exhaust-system branch, a temperature sensor which emits a temperature signal, an SCR catalyst (selective catalytic reduction) and an NH 3 sensor which emits an NH 3 signal and a second SCR catalyst and/or an ASC catalyst (ammonia slip catalyst), and also a separate control unit for evaluating and/or controlling at least one NO x sensor signal, a reducing-agent sensor signal and an exhaust-gas temperature sensor signal, wherein the control unit has a filling-level model of the SCR catalyst, characterised by the following method steps, which are carried out at regular intervals: - measuring the exhaust temperature; - upon exceeding a defined temperature and once a defined NO x mass has flowed through the SCR catalyst, by deliberate, brief exceeding of the slip boundary of the SCR catalyst, abruptly increasing an introduction amount of reducing agent in the exhaust-system branch until the NH 3 sensor detects NH 3 slip; - correcting the filling-level model by overwriting the currently calculated NH 3 filling level of the SCR catalyst by the maximum filling level at the current temperature; - reducing the introduction amount of reducing agent. - measuring the exhaust temperature; - upon exceeding a defined temperature and once a defined NO x mass has flowed through the SCR catalyst, by deliberate, brief exceeding of the slip boundary of the SCR catalyst, abruptly increasing an introduction amount of reducing agent in the exhaust-system branch until the NH 3 sensor detects NH 3 slip; - correcting the filling-level model by overwriting the currently calculated NH 3 filling level of the SCR catalyst by the maximum filling level at the current temperature; - reducing the introduction amount of reducing agent.