An apparatus, system, and method are disclosed for measuring a normalized air-to-fuel ratio. A normalized air-to-fuel ratio is measured by providing an engine control module, providing a first wide-band oxygen sensor in fluid communication with an exhaust stream, adjusting the oxygen pumping current to achieve a stoichiometric balance, detecting the oxygen pumping current, converting the oxygen pumping current to an oxygen balance metric, and communicating the oxygen balance metric to the engine control module. In certain embodiments, the oxygen balance metric may be a volumetric oxygen percentage. In some embodiments, the present invention includes a first and second wide-band oxygen sensor upstream and downstream from an exhaust treatment module.