Abstract: The diagnostic method for a Diesel engine for determining whether this engine, or at least one device connected thereto, is affected by one or several malfunctions negatively influencing the degree of pollution of the exhaust gases produced by this engine, comprises the steps of analyzing the level of an exhaust gas or the evolution of this level, alone or together with the level or evolution of the level of another gas, according to a predetermined state of the engine and characterizing, from said analysis, any malfunction (s) affecting the Diesel engine and/or the devices associated with it. The invention also concerns a device for implementing this method and a computer program executable on the device.