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Two distinctive λ sensors have been developed for a compressed natural gas (CNG) vehicle that complies with OBD-II. As the upstream sensor, there is employed an element which obtains an additional catalyst coating layer to enhance combustion of CH4 and H2 that reaches to the surface of the element. Meanwhile, as the downstream sensor, there is employed an element having an electrode that is intentionally inactive with CH4, to eliminate the temperature dependent chemical reaction of CH4 remaining after the catalyst converter, and to avoid false catalyst monitoring. Use of the above two kinds of λ sensors enables construction of an O2 feedback system for a CNG engine that complies with OBD-II.