NOVELTY - The method involves performing a decomposition step (S1) of applying a wavelet transform on a signal (f) measured on an electrochemical device to decompose the signal into sub-signals represented by wavelet coefficients. A quantization step (S2) of estimating distribution of energy (Ej) of each sub-signal based on the coefficients, is performed. A determining step (S4) of calculating an entropy (ER) for the selected sub-signals containing an information level to determine a signature of the signal. A locating step (S5) of analyzing the signature to locate a fault in an assembly, is performed.    USE - Method for detecting a fault e.g. failure and long life degradation, in an assembly i.e. fuel cell, in an industrial application. Can also be used for a battery and a super-capacitor.    ADVANTAGE - The method enables precise locating of the fault in the assembly without the need for additional sensors in the electrochemical device and a peripheral system, and improves reliability and lifetime of the electrochemical device. The method avoids irreversible degradations and stop of operation of the electrochemical device.    DETAILED DESCRIPTION - The wavelet transform is a discrete wavelet transform. The signal is a voltage signal. INDEPENDENT CLAIMS are also included for the following:    (1) a computer program comprising instructions for performing a method for detecting a fault in an assembly    (2) a recording medium comprising instructions for performing a method for detecting a fault in an assembly    (3) a system for detecting a fault in an assembly.    DESCRIPTION OF DRAWING(S) - The drawing shows a flowchart illustrating a method for detecting a fault in an assembly.    Entropy (ER)    Energy (Ej)    Signal (f)    Decomposition step of applying a wavelet transform on a signal measured on an electrochemical device to decompose the signal into sub-signals (S1)    Quantization step of estimating distribution of energy of each sub-signal (S2)    Determining step of calculating an entropy for the selected sub-signals containing a level of information (S4)    Locating step of analyzing a signature to locate a fault in an assembly (S5)