Catalyst Efficiency Monitor: Notes

The catalyst efficiency monitor uses the rear heated oxygen sensor (HO2S), after the catalyst, to infer the hydrocarbon (HC) efficiency based on the oxygen storage capacity of the catalyst. During monitor operation the PCM calculates the length of the signal while the sensor is switching. Under normal closed loop fuel conditions, high efficiency catalysts have significant oxygen storage. This makes the switching frequency of the rear HO2S very slow and reduces the amplitude, which provides for a shorter signal length. As the catalyst efficiency deteriorates due to thermal and chemical deterioration, its ability to store oxygen declines. The rear HO2S signal begins to switch more rapidly with increasing amplitude and signal length. The predominant failure mode for high mileage catalysts is chemical deterioration (phosphorus deposits on the front brick of the catalyst), not thermal deterioration.