Method of diagnosing a combustion engine assembly, control arrangement, combustion engine assembly, and vehicle

A method of diagnosing an operational performance of a combustion engine assembly, the combustion engine assembly comprising:

• an internal combustion engine, and
• an exhaust system configured to conduct a stream of exhaust gas from the internal combustion engine,
• the exhaust system comprising a SCR catalyst and a NOx sensor arranged downstream of the SCR catalyst,
• and wherein the method comprises the steps of:
• providing a first NOx value (v1) using the NOx sensor during a supply period (p1) of an exhaust additive to the stream of exhaust gas upstream of the SCR catalyst,
• providing a second NOx value (v2) using the NOx sensor during a stop period (p2) of a supply of exhaust additive to the stream of exhaust gas upstream of the SCR catalyst, and
• diagnosing the operational performance of the combustion engine assembly based on a ratio between the first and second NOx values (v1, v2),
• wherein the step of diagnosing the operational performance of the combustion engine assembly comprises the step of setting a fault indicator of at least one of the internal combustion engine and the SCR catalyst if the ratio between the first and second NOx values (v1, v2) is greater than a threshold ratio, and
• wherein the method further comprises steps of:
• providing current operational data of the internal combustion engine during the step of providing the second NOx value (v2), and
• providing a specific NOx content value based on the second NOx value (v2) and on the current operational data, and
• wherein the step of diagnosing the operational performance of the combustion engine assembly further comprises the steps of:
• setting a fault indicator of the SCR catalyst if the ratio between the first and second NOx values (v1, v2) is greater than the threshold ratio and the specific NOx content value is smaller than a threshold value, and
• setting a fault indicator of the internal combustion engine if the specific NOx content value is greater than the threshold value.