A method for diagnosing and/or forecasting the service life of a fuel cell, the method comprising the steps of: applying an excitation signal f n to the fuel cell at a first time t 1, detecting impedances Z n (f n, t 1) at the excitation frequencies of the excitation signal f n at the first time t 1, determining a function Y(f) for the detected impedances Z n (f n, t 1) which contain functional components (Y 1 (f), Y 2 (f), Y 3 (f) and Y 4 (f)) of a predetermined equivalent circuit diagram of the fuel cell, wherein each functional component (Y 1 (f), Y 2 (f), Y 3 (f) and Y 4 (f)) includes at least one equivalent circuit diagram component (R, C), the values of which can be parameterized, determining at least one parameter R(t 1), C(t 1) of at least one equivalent circuit diagram component (R, C) of at least one functional component (Y 1 (f), Y 2 (f), Y 3 (f) and Y 4 (f)) based on the determined function Y(f) matching the impedances Z n (f n, t 1) detected at the first time t 1; applying the excitation signal f n to the fuel cell at a second time t 2, detecting impedances Z n (f n, t 2) at the frequencies of the excitation signal f n at at least one second time t 2, determining the at least one parameter R(t 2), C(t 2) of the at least one equivalent circuit diagram component (R, C) of the at least one functional component (Y 1 (f), Y 2 (f), Y 3 (f) and Y 4 (f)) based on the determined function Y(f) matching the impedances Z n (f n, t 2) detected at the second time t 2; determining a time dependence (R(t), C(t)) for the at least one parameter R(t 1, t 2), C(t 1, t 2) based on the parameters determined for the first and second times t 1, t 2; detecting at least one sign of degradation of a component of the fuel cell assigned to the functional component of the equivalent circuit diagram, based on the time dependence R(t), C(t) of the at least one parameter for at least one functional component (Y 1 (f), Y 2 (f), Y 3 (f) and Y 4 (f)) of the equivalent circuit diagram of the fuel cell, characterized in that a slope of the time dependence R(t), C(t) of an equivalent circuit diagram component (R, C) is determined in order to determine the at least one sign of degradation. applying an excitation signal f n to the fuel cell at a first time t 1, detecting impedances Z n (f n, t 1) at the excitation frequencies of the excitation signal f n at the first time t 1, determining a function Y(f) for the detected impedances Z n (f n, t 1) which contain functional components (Y 1 (f), Y 2 (f), Y 3 (f) and Y 4 (f)) of a predetermined equivalent circuit diagram of the fuel cell, wherein each functional component (Y 1 (f), Y 2 (f), Y 3 (f) and Y 4 (f)) includes at least one equivalent circuit diagram component (R, C), the values of which can be parameterized, determining at least one parameter R(t 1), C(t 1) of at least one equivalent circuit diagram component (R, C) of at least one functional component (Y 1 (f), Y 2 (f), Y 3 (f) and Y 4 (f)) based on the determined function Y(f) matching the impedances Z n (f n, t 1) detected at the first time t 1; applying the excitation signal f n to the fuel cell at a second time t 2, detecting impedances Z n (f n, t 2) at the frequencies of the excitation signal f n at at least one second time t 2, determining the at least one parameter R(t 2), C(t 2) of the at least one equivalent circuit diagram component (R, C) of the at least one functional component (Y 1 (f), Y 2 (f), Y 3 (f) and Y 4 (f)) based on the determined function Y(f) matching the impedances Z n (f n, t 2) detected at the second time t 2; determining a time dependence (R(t), C(t)) for the at least one parameter R(t 1, t 2), C(t 1, t 2) based on the parameters determined for the first and second times t 1, t 2; detecting at least one sign of degradation of a component of the fuel cell assigned to the functional component of the equivalent circuit diagram, based on the time dependence R(t), C(t) of the at least one parameter for at least one functional component (Y 1 (f), Y 2 (f), Y 3 (f) and Y 4 (f)) of the equivalent circuit diagram of the fuel cell, characterized in that a slope of the time dependence R(t), C(t) of an equivalent circuit diagram component (R, C) is determined in order to determine the at least one sign of degradation.