Catalyst Efficiency Monitor - Index Ratio Method (Some 2001-Later)

WARNING: This page does not describe the selected car, but rather 32 other vehicles, including the 2002 Mercury Sable, 2002 Mercury Mountaineer, 2002 Mercury Grand Marquis, 2002 Mercury Cougar, and 2002 Lincoln Town Car. However, it is still accessible from the selected car via links, so may be relevant.

In order to assess catalyst oxygen storage, catalyst monitor counts front HO2S switches during part-throttle, closed-loop fuel conditions after engine is warmed-up and inferred catalyst temperature is within limits. Front switches are accumulated in up to 3 different air mass regions or cells. While catalyst monitoring entry conditions are being met, front and rear HO2S signal lengths are continually being calculated. When required number of front switches has accumulated in each cell, total signal length of rear HO2S is divided by total signal length of front HO2S to compute a catalyst index ratio. An index ratio near zero, indicates high oxygen storage capacity. Therefore, high efficiency. A switch ratio near one, indicates low oxygen storage capacity. Therefore, low HC efficiency. If actual index ratio exceeds threshold index ratio, catalyst is considered faulty. Inputs from ECT or CHT (warm engine), IAT (not extreme ambient temperatures), MAF (greater than minimum engine load), VSS (within vehicle speed window) and TP (at part-throttle) are required to enable Catalyst Efficiency Monitor.

The following are typical switch ratio monitor entry conditions:

Part throttle, maximum rate of change 0.2 volt/0.050 second.
Minimum 330 seconds since start-up at 70°F (21°C).
ECT or CHT is 170-230°F (76.6-110°C).
IAT is 20-180°F (-6 to 82°C).
Engine load 15-40 percent (depending on air mass cell).
Time since entering close loop is 30 seconds.
Vehicle speed is 5-70 MPH.
Fuel level more than 15 percent.
EGR is 1-12 percent.

DTCs associated with this test are DTC P0420 (Bank No. 1 or Y-pipe system), and P0430 (Bank No. 2). Because an exponentially weighted moving average algorithm is used for malfunction determination, up to 6 driving cycles may be required to illuminate MIL during normal customer driving. If KAM is reset or battery is disconnected, a malfunction will illuminate MIL in 2 drive cycles.