In this letter, we propose a fault detection observer for a hydrogen production bioreactor in the occurrence of sensor faults. Based on the bioreactor model in presence of perturbations, we design a residual generator based on a H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞ </sub> observer in order to: (i) detect the occurrence of sensor faults (ii) estimate the glucose and biomass concentrations, and (iii) attenuate the influence of the unknown glucose input and parametric uncertainties. The design conditions, depending only on a gain matrix, can be easily transformed in a semi-definite optimization problem under linear matrix inequalities. The features of the proposed observer are assessed by numerical simulations.