In this paper, the energy diagnosis improvement of an existing steam power plant is faced on the basis of the Lost work Impact Formula, developed in the ambit of thermo-economics and presented in a previous paper. Three strategies for improving diagnostic accuracy are discussed and exemplified. Some cases of simultaneous malfunctioning of two components are introduced, to test the accuracy of energy recovery evaluation when intrinsic malfunction affects more than one component. The improved results show that the effect of the functional decay can be quantified with sufficient approximation also in that case where the first diagnostic attempt did not give meaningful answers.