Recent Federal and California legislation have mandated major improvements in emission control. Tailpipe HC emission must be decreased an order of magnitude for the California Ultra Low Emission Vehicle (ULEV) standard. Present feedback A/F* control systems employ a Heated Exhaust Gas Oxygen sensor (HEGO sensor) upstream of the catalyst to perform A/F feedback control. Limitations on the ultimate accuracy of these switching sensors are well known. To overcome these limitations a linear Universal Exhaust Gas Oxygen sensor (UEGO sensor) placed downstream from the minicatalyst is employed to attain improved A/F control and therefore, higher three-way catalyst (TWC) conversion efficiency. This configuration was granted a patent in 1992 (1**).This study compares performance differences between the two feedback control systems on a Ford Mustang. In initial studies both the UEGO and HEGO sensors were compared at the midposition location downstream of a minicatalyst. Although several HEGO calibrations were used, the UEGO sensor with Proportional-Integral (PI) control provided lower tailpipe emissions. Control loop frequency loss was minimal and the A/F control point stabilized by the filtering action of the minicatalyst.Control loop behavior was tested with aged minicatalysts and UEGO sensors to determine if a practical increase in the loop gain could be attained to compensate for catalyst aging.In conclusion, the midposition UEGO sensor feedback control, with further developement, appears to be a promising combination for future emission reduction.