Operating and diagnostic method for an SCR exhaust-gas aftertreatment system

1. 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.

2. The method according to claim 1 , further comprising:

returning to the normal operating mode if the signal of the exhaust gas sensor falls below a predefinable third limit value lying below the first limit value within the predefinable period after the increase of the set urea dosing rate.

3. The method according to claim 2 , further comprising:

diagnosing a malfunctioning urea dosing device if the signal of the exhaust gas sensor remains in a predefinable value range including the first limit value in the predefinable period.

4. The method according to claim 3 , wherein the predefinable value range is limited by the second limit value at an upper end and by the third limit value at the a lower end.

5. The method according to claim 1 , further comprising:

determining a catalytic converter characteristic characterizing an updated nitrogen conversion activity in the diagnostics operating mode.

6. The method according to claim 5 , further comprising:

determining a deviation of the catalytic converter characteristic determined in the diagnostic mode from a predefined reference value; and

generating a malfunction message if the deviation is larger than a predefined difference value.

7. The method according to claim 6 , further comprising:

ending the internal combustion engine operation with increased nitrogen oxide untreated emission and returning to the normal operating mode if the signal of the exhaust gas sensor exceeds a predefinable fourth limit value after the increase of the nitrogen oxide untreated emission.