Circuit Description

PCM monitors the Heated Oxygen Sensor (HO2S) activity for 100 seconds. During this period, PCM counts the number of times that the HO2S switches from rich to lean and lean to rich and adds the amount of time it took to complete all transitions.

With this information, an average time for all transitions can be determined. The PCM then divides the rich to lean average by the lean to rich average to obtain a ratio. If HO2S ratio is not within range, DTC will set.