The guideline deals with measuring concentrations of carbon monoxide emissions. Alternativly, the method described in this guideline can be used, which is based on gas-chromatographic separation and catalytic reduction of the CO to methane (CH4). Since no other practicable reference methods which are independent of calibration gases are currently available, this measurement method can be used as a convention method and by tracing it back to national reference standards, it can also be used as a reference method.