Thermoeconomic diagnostic approaches have proved to be effective in quantifying additional exergy consumption and costs, whereas they demonstrated to be less appropriate in the search of malfunction causes. With the intent of overcoming this limitation the Characteristic Curve Method was suggested by Toffolo and Lazzaretto, which is based on the idea that every malfunction leaves an undeletable trace in the system: the modification of the component characteristic curve. An exergetic index was found to be very suitable to highlight this trace, and demonstrated consequently to be very effective in identifying the causes of malfunctions. In this paper the method is applied to an existing cogeneration plant, that is simulated using a commercially available code. Simulation errors due to the iterative criterion used by the simulator to find convergence are considered in the analysis, and the reliability of the method under these conditions checked. The example of application is used to show all the calculation steps to be performed, and the way of reading the results in order to properly detect causes of malfunctions. This helps avoid misleading conclusions and demonstrates the practical and easy application of the method when commercial simulator codes are used.