Catalyst Efficiency Monitor: Description

WARNING: This page does not describe the selected car, but rather 32 other vehicles, including the 2003 Mercury Sable, 2003 Mercury Mountaineer, 2003 Mercury Marauder, 2003 Mercury Grand Marquis, and 2003 Lincoln Town Car. However, it is still accessible from the selected car via links, so may be relevant.

Catalyst Efficiency Monitor uses an oxygen sensor before and after catalyst to infer hydrocarbon efficiency based on oxygen storage capacity of catalyst. Under normal, closed-loop fuel conditions, high efficiency catalysts have significant oxygen storage. This makes switching frequency of rear Heated Oxygen Sensor (HO2S) very slow and reduces amplitude of those switches as compared to switching frequency and amplitude of front HO2S. As catalyst efficiency deteriorates due to thermal and/or chemical deterioration, its ability to store oxygen declines. Post-catalyst or downstream HO2S signal begins to switch more rapidly with increasing amplitude, approaching switching frequency and amplitude of pre-catalyst or upstream HO2S.

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

All vehicles utilize a Federal Test Procedure (FTP) based catalyst monitor. Meaning catalyst monitor must run during a standard FTP emission test. This differs from a 20 second steady state catalyst monitor used in 1994-96 vehicles. Currently, 2 slightly different versions of catalyst monitor are utilized; Switch Ratio Method and Index Ratio Method. Beginning with 2001 model year, both versions will continue to be used in subsequent model years.