Catalyst Efficiency Monitor: Notes

The Catalyst Efficiency Monitor uses an oxygen sensor before and after the catalyst 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 rear heated oxygen sensor (HO2S) very slow and reduces the amplitude of those switches as compared to the switching frequency and amplitude of the front HO2S. As the catalyst efficiency deteriorates due to thermal and/or chemical deterioration, its ability to store oxygen declines. The post-catalyst or downstream HO2S signal begins to switch more rapidly with increasing amplitude, approaching the switching frequency and amplitude of the pre-catalyst or upstream HO2S.

All vehicles use a Federal Test Procedure based catalyst monitor. This simply means that the catalyst monitor must run during a standard Federal Test Procedure emission test. This differs from the 20-second steady state catalyst monitor used in 1994 through some 1996 vehicles. Currently, the 2 slightly different versions of the catalyst monitor that are used are the Switch Ratio method and the Index Ratio method. Beginning with the 2001 model year and beyond, both versions will continue to be used in subsequent model years.