In this paper, a nonlinear observer based on a Takagi Sugeno approach is developed and applied to a PEMFCS (Proton Exchange Membrane Fuel Cell Stack) in the case of unmeasurable premise variables. The FC model focuses on the auxiliary elements associated to the stack. The challenge is to construct a TS fuzzy observer such that it can asymptotically estimate the states of the considered nonlinear systems despite the unmeasurable premise variables. The results are validated through the use of the professional software AMESim using a co-simulation with Matlab-Simulink.