High Temperature PEM fuel cells are able to withstand a substantial amount of CO poisoning while still maintaining stable output. High concentrations of CO will however degrade performance and can lead to instability. This paper presents a detailed study on the impact of CO poisoning on the performance of High Temperature PEM fuel cells. A dedicated test setup was constructed to perform both steady state and dynamic analysis on the fuel cell under a wide range of operating conditions and variations in CO content. The drop in performance captured in the polarization curves is modeled using a simple circuit model with a dedicated fault element. The captured impedance spectra from the electrochemical impedance spectroscopy (EIS) tests provide insight to the changes in the electrochemical circuit parameters during CO poisoning that can be used to track the fuel cell state of health. The results from this study will allow for the development of a rapid diagnostic control system that can be implemented in the power conditioning unit of the fuel cell. © 2013 IEEE.