Catalyst Efficiency Monitor Overview

WARNING: This page does not describe the selected car, but rather 32 other vehicles, including the 2005 Mercury Sable, 2005 Mercury Mountaineer, 2005 Mercury Monterey, 2005 Mercury Montego, and 2005 Mercury Mariner. However, it is still accessible from the selected car via links, so may be relevant.

The catalyst efficiency monitor uses pre 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 pre-catalyst 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 pre-catalyst HO2S (A).

NOTE: HO2S (A) and (B) locations may vary depending on exhaust system configuration. For additional information on CATALYST EFFICIENCY MONITOR and EXHAUST SYSTEM .