Membrane drying, fuel cell flooding, and anode catalyst poisoning by carbon monoxide are investigated on Hydrogenics production-type proton exchange membrane fuel cell (PEMFC) stacks similar to the stacks used in Hydrogenics HyPM 10-kW fuel cell power modules. Changes in fuel cell voltage and impedance with time are presented for each type of fault, the fuel cell stacks being controlled in galvanostatic mode. This study shows that these PEMFC stack faults can be differentiated by their impedance responses while fuel cell voltage monitoring alone is insufficient to distinguish between failure types. Membrane drying leads to an increase in the fuel cell impedance magnitude and phase angle at all frequencies studied. Fuel cell flooding leads to an increase in the impedance magnitude at low frequencies (f < 10 Hz) and to a decrease in the impedance phase angle at frequencies less than 100 Hz. Anode catalyst poisoning by CO is characterized by an increase in the fuel cell impedance magnitude at frequencies less than a few hundred Hz. For this fault, the impedance phase angle decreases within a large frequency range and is characterized by a minimum value appearing at 20-25 Hz at moderate current density. (c) 2006 The Electrochemical Society.