Circuit Description

PCM continuously monitors the HO2S activity for 100 seconds. During this period, PCM counts the number of times the HO2S responds from rich to lean and from lean to rich, and adds the amount of time it took to complete the transition. With this information, an average time for all transitions can be determined. If the average response time is too slow, DTC will set.

A lean to rich transition is determined when the HO2S voltage changes from less than 300 mV to greater than 600 mV. A rich to lean transition is determined when the HO2S voltage changes from greater than 600 mV to less than 300 mV. A sensor that responds too slowly is most likely defective and should be replaced.