In recent years, the EPA has implemented a requirement for monitoring the air fuel ratio balance in multi-cylinder engines such that an imbalance may not be so great as to cause the tailpipe emissions level to exceed 1.5 times the nominal emissions standard. Such imbalances may be the result of production fuel injector variation, contamination, leaks, or other malfunctions which cause the air or fuel rate to vary across the cylinders controlled by a single oxygen sensor. For many diagnostic systems that rely on the signal from the oxygen sensor, to achieve compliance to the new diagnostic standard, the sensor must see the signal from each cylinder equally. The aftertreatment system must also be robust to individual cylinder air fuel ratio variation. This paper introduces the concept of catalyst zone flow, a condition in which different cylinders of a multi-cylinder engine use different portions of the catalyst brick. When the air fuel ratio is maldistributed, different portions of the catalyst brick may be operating with different air fuel ratios, despite good overall control of the mean air fuel ratio. This condition can result in poor three-way catalyst efficiency. A CAE metric called the zone flow index is defined, which quantifies the level of velocity overlap that occurs within the catalyst brick for each cylinder as it blows down. A highly close coupled catalyst brick test case is examined, with measurements that support the existence of catalyst zone flow. The correlation to CFD is discussed, and a patented strategy to reduce catalyst zone flow is introduced as one means of addressing the problem.