A method of diagnosing an engine system including an engine and an exhaust aftertreatment system. The engine system comprises a plurality of replaceable engine system components. The method comprises maintaining a database with component data comprising, for each one of the replaceable engine system components, a deterioration efficiency parameter correlating the deterioration status of the replaceable engine system component to emission reducing efficiency, and a utility component parameter comprising component exchangeability data; determining the deterioration status of each one of the replaceable engine system components; estimating the emission reducing efficiency for each one of the replaceable engine system components in response to the determined deterioration status and the deterioration efficiency parameter; based on known relation between component exchangeability data and the resulting emission reducing efficiency of a potential exchange of the replaceable engine system component, identifying a replaceable engine system component to exchange.