Error states in PEMFC stacks typically cause distortions of the local current distribution and the cell voltage. Measuring those properties can be used for identification of those error states, e.g., membrane dry-out, channel blockage, reactant media contamination, local starvation, gross starvation, overall stack starvation and pinholes. Two techniques for online diagnosis are introduced, measuring the current density distribution by a printed circuit board, and measuring the local cell voltage by a multipoint cell voltage monitoring. A non-invasive experimental setup is proposed for both techniques to gain reliable measurement data. The relationship of the cell voltage distribution over the stack and the current density distribution within the cells is discussed and used to discriminate different error states. Experimental measurement data of those techniques is shown for several common error states. Finally, a categorization of error states is proposed, based on their characteristic patterns of disturbed local current density and cell voltage.