Diagnosis techniques are usually adopted in energy systems to prevent anomalies that can cause, if not repaired, failures (Zaleta 1997). The aim of thermoeconomic diagnosis is different in that it is oriented to plant energy savings while the plant is in operation. In particular, it focuses on discovering reductions in system efficiency (detection of anomalies), locating where these inefficiencies have occurred, and understanding their causes. The solution of this problem is not banal, due to ‘collateral effects’. In fact, when the efficiency of a component decreases because of an anomaly, the efficiencies of the other components generally vary. Moreover, if some settings are not complied with, the control system intervenes and commands an adjustment, which changes the effects of the anomaly on the components.This paper proposes an approach particularly helpful for the diagnosis of complex energy systems. With this approach, the system is first divided into macro-components. A zooming strategy allows one to focus attention on a small part of the system. Then a detailed analysis of a few macro-components (the really malfunctioning ones) is conducted. Such an analysis is based on the principle of eliminating the contributions of the main collateral effects, i.e. the efficiencies of the components dependent on their operating conditions and control system interventions.In this paper, the procedure is presented and applied to a combined cycle, composed of two gas turbines, two HRSGs and a steam turbine.