Method for diagnosing a selective catalytic reduction catalyst

A method for determining an ammonia (NH3) storage capacity of a selective catalytic reduction (SCR) catalyst of an exhaust aftertreatment system of an internal combustion engine, wherein the exhaust aftertreatment system comprises a SCR catalyst, an reductant injector arranged upstream of the SCR catalyst for injecting a reductant into an exhaust gas, and an exhaust gas sensor arranged downstream of the SCR catalyst, which exhaust gas sensor being cross-sensitive to nitrogen oxide (NOx) and NH3 present within the exhaust gas leaving the SCR catalyst, wherein the method comprises the steps of:

• ensuring that a SCR catalyst NH3 storage level is substantially below a current maximal NH3 storage capacity of the SCR catalyst;
• initiating over-injection of reductant by the injector;
• stopping reductant injection in response to registering, by the exhaust gas sensor, an increasing NOx+NH3 emission level and an NOx+NH3 emission level exceeding a predefined threshold value, and when the reduction injection is stopped recording an output signal of the exhaust gas sensor until an indication of a minimal negligible SCR catalyst NH3 storage level is determined;
• and
• calculating a SCR catalyst NH3 storage capacity on a basis of the recorded output signal, wherein the storage capacity is calculated by integrating a difference between an estimated or recorded level of nitrogen oxide (NOx) having entered the SCR catalyst and a recorded level of nitrogen oxide (NOx) having been emitted from the SCR catalyst.