In engineering, the word diagnosis identifies procedures for detecting the presence of anomalies in systems, locating where they have occurred and quantifying them.Thermoeconomic diagnosis procedures are based on the productive representation of systems. This is a mathematical expression of the role played by each component in the whole plant, made by defining its fuels and products in terms of exergy flows. This is called the productive structure.The details of a productive structure are at two different levels, one with respect to the number of components and the other with respect to the number of productive flows. The first one is selected according to the accuracy desired in the location of the anomalies. The higher the number of components is, the higher is the accuracy. Once the components are identified, the number of productive flows can be increased by separating exergy into its components (Tsatsaronis et al., 1990) or by introducing fictitious flows (Frangopoulos 1987, Von Spakovsky and Evans 1990). This decision facilitates the assessment of the nature of the anomaly (thermal, mechanical or chemical), but also affects the results of the thermoeconomic analysis, even when it is adapted for diagnosis purposes.In this paper the effects of these decisions on the results of the thermoeconomic diagnosis is investigated. A particularly sensitive test, obtained by simulating an anomaly in the HRSG of a combined cycle plant, is considered.