Method of monitoring CO concentrations in hydrogen feed to a PEM fuel cell

A method of operating a H-O fuel cell system comprising the principal steps of (1) providing a hydrogen-rich fuel gas having a first CO content sufficient to poison the fuel cell's anode, and (2) injecting oxygen into said fuel gas upstream of said fuel cell to oxidize said CO with O in the presence of said hydrogen to produce a fuel stream for said fuel cell which has a second CO content less than said first content that will not poison said fuel cell.