Patent ID: 8499545

Claim:
A method for operating and diagnosing an internal combustion engine SCR exhaust gas aftertreatment system that has an SCR catalytic converter with ammonia storage capacity, and an exhaust gas sensor that is sensitive to nitrogen oxide and ammonia arranged downstream of the SCR catalytic converter; wherein the method comprising: measuring a signal of said exhaust gas sensor; operating the internal combustion engine SCR exhaust gas aftertreatment system in a normal operating mode by setting a urea dosing rate to a normal operating mode dosing rate; supplying ammonia to the SCR catalytic converter corresponding to a filling level model for ammonia stored in the SCR catalytic converter, in such a way that the ammonia filling level of the SCR catalytic converter is held at least at a predefinable nominal value or value range; detecting that the signal of the exhaust gas sensor exceeds a first predefinable limit value; and interrupting the normal operating mode of the internal combustion engine SCR exhaust gas aftertreatment system; and increasing the a urea dosing rate by a predefinable amount compared to the normal operating mode dosing rate; detecting, within a predefined period after increasing the urea dosing rate, that the signal of the exhaust gas sensor increases over a predefinable second limit value exceeding the first predefinable limit value within the predefinable period; and changing the operation of the internal combustion engine SCR exhaust gas aftertreatment system to a diagnostic operating mode, wherein the diagnostic operating mode includes controlling the internal combustion engine to produce an increased nitrogen oxide untreated emission compared to a nitrogen oxide untreated emission in normal operation of the internal combustion engine; and reducing the urea dosing rate in chronological synchronism to the increase of nitrogen oxide untreated emission or setting the urea dosing rate to zero.