A sub-oxygen sensor (S2) placed downstream, at the outlet of the catalytic converter, is monitored for switching frequency and compared to the switching frequency of the main oxygen sensor (S1), placed upstream of the catalyst. The oxidation efficiency of the catalyst can be determined by comparing the switching frequency of these two sensors. As the catalyst conversion efficiency declines, the switching frequency of sensor 2 increases, approaching that of sensor 1. In addition to being used for diagnostics, sensor 2 also assists in maintaining optimum fuel control when the catalyst begins to degrade.
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