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 is disclosed, the combustion engine assembly comprising a combustion engine, and an exhaust system. The exhaust system comprises a SCR catalyst and a NOx sensor arranged downstream of the SCR catalyst. The method comprises the steps of providing a first NOx value (v) using the NOx sensor during a supply period (p) of an exhaust additive, providing a second NOx value (v) using the NOx sensor during a stop period (p) of a supply of exhaust additive, and diagnosing the operational performance of the combustion engine assembly based on a ratio between the first and second NOx values (v, v). The present disclosure further relates to a computer program, a computer-readable medium, a control arrangement, a combustion engine assembly, and a vehicle.