- In fuel cell systems reformers are used to obtain hydrogen from fuels such as natural gas producing a relatively high concentration of carbon monoxide as byproduct (up to 500ppm at start-up). This poses a serious problem to the operation of PEM fuel cell stacks reducing its active area due to poisoning; and thus its terminal voltage and output power. This paper presents a cost effective solution for the CO poisoning problem, based on a low cost, reduced power rating converter. It is shown that the proposed system allows the operation of the stack with contaminated fuel for extended periods of time with minimal output power degradation. Additionally the proposed system has the advantage of being an add-on solution that can be connected to existing power conditioner units, being rated only for 10% of the nominal power of the main converter. 2008 IEEE.