Circuit Description
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PCM monitors Heated Oxygen Sensor (HO2S) activity for 100 seconds. During this period, PCM counts number of times that HO2S switches from rich-to-lean and lean-to-rich and adds amount of time it took to complete all transitions.
With this information, an average time for all transitions can be determined. PCM then divides rich-to-lean average by lean-to-rich average to obtain a ratio. If HO2S ratio is not within range, DTC will set.