In comparison to state of the art fuel cell stack monitoring techniques, where for reliability and durability reasons either single cell or cell-block voltages are monitored separately, the new approach derives information about critical cell and stack status from the stack sum voltage only. The motivation for the development of such a technology is to establish a strongly simplified and low cost stack diagnosis unit. In comparison to the cell voltage monitoring (CVM) technology, where up to several hundreds voltage channels have to be measured separately or at least in pairs, the effort for wiring, contacting and instrumentation can be reduced dramatically. Critical cell operation occurs, if e.g. low air stoichiometry causes a sharp drop of voltage at a certain cell current. If in such case the current is superimposed by a small amplitude signal with specific frequency pattern, then the system response (i.e. stack voltage) will be distorted in the frequency domain. Particularly, this means that even if only one single cell is in a critical operation mode, it would cause distorted frequency fractions extractable from the entire stack voltage. Since this approach is related to the distortion of the frequency pattern only, noise and EMC issues are not significantly influencing the measuring quality. The instrumentation and processing effort is rather low and can be realized with a low cost DSP board. Measurement results for different critical stack operation modes, e.g. operation at low air or low hydrogen stoichiometry, with their correlation to the frequency distortion will be described and discussed. (c) 2006 Elsevier B.V. All rights reserved.