This paper presents an original approach used for the reconstruction of a current density on a fuel cell simulator. It is based on the 3D reconstruction of the electrical current density from external magnetic field measurements. As a first step, we simulate conductivity faults in a fuel cell by using an electro- kinetic model. The current distribution is therefore locally modified. This modification is observed on the calculated magnetic field on some sensor positions. The fault current is decomposed as a linear combination of current density vectors which represent a basis. We compute a magnetic field vector for each current vector. For any magnetic signature of a fuel cell we find an associated current distribution thanks to the truncated singular values decomposition (SVD). Some works were interested on this kind of method [1], [2] and [3]. This approach has been first validated on a stack simulator in order to check the feasibility of the system (Fig. 1 and Fig. 2). This stack simulator is geometrically representative of a real stack and consists in a truncated graphite bloc to simulate a fault. Figs. 3-a and 3-b allow comparing the current density that was imposed in the simulator against the current density that was computed from the external magnetic field measurement. Although the resolution is not perfect, the main component of the current is observable.