Method for diagnosing a fuel-cell stack comprising the following steps: - acquiring the electric current (I_m) produced by the fuel-cell stack at a first time, - acquiring the voltage (U_m) across the terminals of the fuel-cell stack and the impedance (Z HF _m) of the fuel-cell stack at said first time, - estimating, under normal operating conditions, from said measured current injected into a simplified first model (M1_j) relating the variation in the voltage as a function of the current and the variation in the electrical impedance as a function of the current, a theoretical voltage (U_th_1) across the terminals of the stack at said first time and the theoretical electrical impedance (Z HF _th_1) of the stack at said first time, - determining a first residue between said measured voltage (U_m) and said determined theoretical voltage (U_th_1), - determining a second residue between said measured impedance (Z HF _m) and said determined theoretical electrical impedance (Z HF _th_1), - determining at least one type of fault using a second simplified model (M2_J) configured to relate the values of the two residues to a type of fault, - characterized in that: - the first simplified model (M1_j) and said second simplified model (M2_J) are each generated from a physical model of the fuel-cell stack incorporating a fuel-cell stack degradation module that takes into account the current and the voltage that are applied to said fuel-cell stack in a period prior to said first time, - said first simplified model (M1_j) is obtained by applying, as input to said physical model of the fuel-cell stack, under nominal operating conditions, a plurality of different current operating points (I_k), - said second simplified model (M2_J) is obtained by applying, as input to said physical model of the fuel-cell stack, under fault-affected operating conditions, a plurality of different current operating points (I_k). - acquiring the electric current (I_m) produced by the fuel-cell stack at a first time, - acquiring the voltage (U_m) across the terminals of the fuel-cell stack and the impedance (Z HF _m) of the fuel-cell stack at said first time, - estimating, under normal operating conditions, from said measured current injected into a simplified first model (M1_j) relating the variation in the voltage as a function of the current and the variation in the electrical impedance as a function of the current, a theoretical voltage (U_th_1) across the terminals of the stack at said first time and the theoretical electrical impedance (Z HF _th_1) of the stack at said first time, - determining a first residue between said measured voltage (U_m) and said determined theoretical voltage (U_th_1), - determining a second residue between said measured impedance (Z HF _m) and said determined theoretical electrical impedance (Z HF _th_1), - determining at least one type of fault using a second simplified model (M2_J) configured to relate the values of the two residues to a type of fault, - characterized in that: - the first simplified model (M1_j) and said second simplified model (M2_J) are each generated from a physical model of the fuel-cell stack incorporating a fuel-cell stack degradation module that takes into account the current and the voltage that are applied to said fuel-cell stack in a period prior to said first time, - said first simplified model (M1_j) is obtained by applying, as input to said physical model of the fuel-cell stack, under nominal operating conditions, a plurality of different current operating points (I_k), - said second simplified model (M2_J) is obtained by applying, as input to said physical model of the fuel-cell stack, under fault-affected operating conditions, a plurality of different current operating points (I_k). - the first simplified model (M1_j) and said second simplified model (M2_J) are each generated from a physical model of the fuel-cell stack incorporating a fuel-cell stack degradation module that takes into account the current and the voltage that are applied to said fuel-cell stack in a period prior to said first time, - said first simplified model (M1_j) is obtained by applying, as input to said physical model of the fuel-cell stack, under nominal operating conditions, a plurality of different current operating points (I_k), - said second simplified model (M2_J) is obtained by applying, as input to said physical model of the fuel-cell stack, under fault-affected operating conditions, a plurality of different current operating points (I_k).