A gas injection method for treating an electrochemical fuel cell stack assembly, the fuel cell stack assembly being repeatedly injected with an oxidizing gas at critical locations along the fuel cell stack assembly so that the fuel supply and the oxidizing gas will chemically react to reduce at least one harmful contaminant within the fuel supply. The preferred gas injection method treats a fuel cell stack assembly to reduce the debilitating effects of extraneous carbon monoxide within the fuel supply and thus preserves the efficient operation of the fuel cell stack assembly.