Catalyst Efficiency Monitor: Notes

NOTE: The primary failure mode for high mileage catalysts is chemical deterioration (phosphorus deposition on the front brick of the catalyst), not thermal deterioration as is often assumed.

Catalyst Efficiency Monitor Overview - The catalyst efficiency monitor uses a precatalyst and post-catalyst heated oxygen sensors (HO2S) to infer the hydrocarbon (HC) efficiency based on the oxygen storage capacity of the catalyst. Under normal, closed loop fuel conditions, high efficiency catalysts have significant oxygen storage. This makes the switching frequency of the post-catalyst HO2S (B) very slow and reduces the amplitude of those switches as compared to the switching frequency and amplitude of the precatalyst HO2S (A). As catalyst efficiency deteriorates due to thermal and chemical deterioration, its ability to store oxygen declines. The post-catalyst HO2S (B) signal begins to switch more rapidly with increasing amplitude, approaching the switching frequency and amplitude of the precatalyst HO2S (A).

Fig 1: High Catalyst Efficiency

Fig 2: Low Catalyst Efficiency

NOTE: The primary failure mode for high mileage catalysts is chemical deterioration (phosphorus deposition on the front brick of the catalyst), not thermal deterioration as is often assumed.