Note. For circuit reference, see WIRING DIAGRAMS article.
Circuit Description
PCM reads Heated Oxygen Sensor (HO2S) activity for 100 seconds. During this period, PCM counts the number of Lean/Rich (L/R) and Rich/Lean (R/L) switches.
With this information, an average time for all transitions can be determined. PCM then divides R/L average by L/R rich average to obtain a ratio. If HO2S ratio is not within range, DTC will set.