The health of power transformer is important for the reliability of electrical power grid. There are various methods of evaluating the frequency spectrum to confirm the presence of an incipient fault. So, the accuracy of FRA method suggested by many researchers for predictive defections, has been discussed; and the robustness (efficiency, simplicity and fast diagnosis) of the proposed 2LFW as a substitution diagnostic process has been detailed and proved. It should be noted that the proposal is general and can be applied to different transformer, without any comparison to a reference signal. Furthermore the diagnosis can be done even for transformers in operation mode (in service), thus avoiding economical losses.