The system comprises a stack of proton exchange membrane (PEM) H2-O2 cells each comprising principally a PEM having an anode and a cathode affixed to opposing two surfaces of it, a flow channel adjacent the anode for lowering hydrogen into contact with the anode, and a second flow channel adjacent the cathode for flowing an oxygen-bearing gas into contact with the cathode. The stack has a first end cell at one end of the stack, a second end cell at the opposite end of the stack and several additional cells intermediate the end cells. An oxygen feed manifold supplies oxygen to the cells, and a hydrogen feed manifold supplies hydrogen to the cells. An external electrical device having a variable electrical load is connected to the end cells for receiving electrical current from the stack for operating the device. The fuel cell system is improved by an auxiliary PEM H2-O2 fuel cell having third and fourth flow channels respectively receiving hydrogen and oxygen from the respective feed manifolds, under the same temperature, pressure, CO concentration, and humidity conditions as the cells of the stack.    A pair of terminals for withdrawing electrical current from the auxiliary cell are connected to a constant electrical load. A voltage sensing device senses voltage fluctuations across the constant load over a predetermined time interval and outputs a voltage signal having a behaviour pattern indicative of such voltage fluctuations. A current sensing device senses current fluctuations through the constant load over a predetermined time interval and outputs a current signal having a behaviour pattern indicative of such current fluctuations. A monitor receiving the voltage and current signals compares the behaviour patterns of such signals to predetermined reference voltage and current signatures indicative of the physical condition of the auxiliary cell, and hence the stack, and outputs a warning signal when the early stages of an undesirable operating condition is indicated.    USE -   For monitoring PEM fuel cell stacks, and initiating corrective action when undesirable stack operating conditions are indicated.    ADVANTAGE -   Advanced warning of impending undesirable stack operating conditions, excessive CO concentration, prevents damage to stack and allows early corrective action to be initiated.