Method for detecting faults and/or degradation in a fuel cell assembly by analysis of the power spectrum density of a voltage signal output by the fuel cell assembly during its steady-state operations, the method comprising the following step: acquiring a voltage signal output by the fuel cell assembly as a function of time; the method being characterized by comprising the following steps: identifying at least one voltage segment s i in the acquired voltage signal with i= 1,...,n, n ∈ N and n ≥ 1, each voltage segment s i being a collection of voltage measurements in a i -th time interval in which the fuel cell assembly is considered in a steady-state condition; detrending each one of the at least one voltage segment s i to remove a slow-varying component, obtaining a de-trended voltage segment s i '; estimating the Power Spectrum Density, PSD, of each one of the at least one voltage segment s i ' and its PSD peaks P k i at characteristic frequencies f k of the fuel cell system assembly, with k= 1,...,M, M ∈ N and M ≥ 1; computing a test statistic T i = T i P k i R k for each one of the at least one voltage segment s i, as a function of its PSD peaks magnitude values P k i and of reference values R k, each reference value R k being a magnitude of a peak at the characteristic frequency f k of the fuel cell assembly during normal steady-state operation; checking whether the test statistic T i is greater than a predetermined threshold γ: if the test statistic T i is lower than or equal to the predetermined threshold γ, recognising that the fuel cell assembly is in normal steady-state operation during the corresponding i -th time interval; if the test statistic T i is greater than the predetermined threshold γ, recognising that fault and/or degradation occurred in the fuel cell system during the corresponding i -th time interval. acquiring a voltage signal output by the fuel cell assembly as a function of time; the method being characterized by comprising the following steps: identifying at least one voltage segment s i in the acquired voltage signal with i= 1,...,n, n ∈ N and n ≥ 1, each voltage segment s i being a collection of voltage measurements in a i -th time interval in which the fuel cell assembly is considered in a steady-state condition; detrending each one of the at least one voltage segment s i to remove a slow-varying component, obtaining a de-trended voltage segment s i '; estimating the Power Spectrum Density, PSD, of each one of the at least one voltage segment s i ' and its PSD peaks P k i at characteristic frequencies f k of the fuel cell system assembly, with k= 1,...,M, M ∈ N and M ≥ 1; computing a test statistic T i = T i P k i R k for each one of the at least one voltage segment s i, as a function of its PSD peaks magnitude values P k i and of reference values R k, each reference value R k being a magnitude of a peak at the characteristic frequency f k of the fuel cell assembly during normal steady-state operation; checking whether the test statistic T i is greater than a predetermined threshold γ: if the test statistic T i is lower than or equal to the predetermined threshold γ, recognising that the fuel cell assembly is in normal steady-state operation during the corresponding i -th time interval; if the test statistic T i is greater than the predetermined threshold γ, recognising that fault and/or degradation occurred in the fuel cell system during the corresponding i -th time interval. identifying at least one voltage segment s i in the acquired voltage signal with i= 1,...,n, n ∈ N and n ≥ 1, each voltage segment s i being a collection of voltage measurements in a i -th time interval in which the fuel cell assembly is considered in a steady-state condition; detrending each one of the at least one voltage segment s i to remove a slow-varying component, obtaining a de-trended voltage segment s i '; estimating the Power Spectrum Density, PSD, of each one of the at least one voltage segment s i ' and its PSD peaks P k i at characteristic frequencies f k of the fuel cell system assembly, with k= 1,...,M, M ∈ N and M ≥ 1; computing a test statistic T i = T i P k i R k for each one of the at least one voltage segment s i, as a function of its PSD peaks magnitude values P k i and of reference values R k, each reference value R k being a magnitude of a peak at the characteristic frequency f k of the fuel cell assembly during normal steady-state operation; checking whether the test statistic T i is greater than a predetermined threshold γ: if the test statistic T i is lower than or equal to the predetermined threshold γ, recognising that the fuel cell assembly is in normal steady-state operation during the corresponding i -th time interval; if the test statistic T i is greater than the predetermined threshold γ, recognising that fault and/or degradation occurred in the fuel cell system during the corresponding i -th time interval. if the test statistic T i is lower than or equal to the predetermined threshold γ, recognising that the fuel cell assembly is in normal steady-state operation during the corresponding i -th time interval; if the test statistic T i is greater than the predetermined threshold γ, recognising that fault and/or degradation occurred in the fuel cell system during the corresponding i -th time interval.