A method and a control system are provided for diagnosing a diesel combustion catalyst that is located in an exhaust line within a diesel engine system. The method comprising providing an unburned fuel mass flow through the diesel oxidation catalyst, determining the oxidation heat release rate which is related to the exothermic oxidation reactions of the unburned fuel into the diesel oxidation catalyst, integrating the determined oxidation heat release rate on a time interval, integrating the unburned fuel mass flow on the same time interval, dividing the integrated value of oxidation heat release rate and the integrated value of unburned fuel mass flow for determining an efficiency index (DOI) of the diesel oxidation catalyst.