Index Ratio Method
WARNING: This page does not describe the selected car, but rather 30 other vehicles, including the 2004 Mercury Sable, 2004 Mercury Mountaineer, 2004 Mercury Monterey, 2004 Mercury Marauder, and 2004 Mercury Grand Marquis. However, it is still accessible from the selected car via links, so may be relevant.
- In order to assess catalyst oxygen storage, the catalyst monitor counts front HO2S switches during part throttle, closed-loop fuel conditions after the engine is warmed-up and inferred catalyst temperature is within limits. Front switches are accumulated in up to three different air mass regions or cells. While catalyst monitoring entry conditions are being met, the front and rear HO2S signal lengths are continually being calculated. When the required number of front switches has accumulated in each cell, the total signal length of the rear HO2S is divided by the total signal length of the front HO2S to compute a catalyst index ratio. An index ratio near 0.0 indicates high oxygen storage capacity; hence high HC efficiency. A switch ratio near 1.0 indicates low oxygen storage capacity; hence low HC efficiency. If the actual index ratio exceeds the threshold index ratio, the catalyst is considered failed.
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 the Catalyst Efficiency Monitor.
Typical Index Ratio Monitor Entry Conditions:
- Minimum 330 seconds since start-up at 70° F (21°C)
- Engine coolant temperature is between 170° F (76.6°C) and 230°F (110°C)
- Intake air temperature is between 20°F (-6°C) and 180°F (82°C)
- Time since entering close loop is 30 seconds
- Inferred Rear HO2S sensor temperature of 900° F (482° C)
- EGR is between 1 and 12%
- Part throttle, maximum rate of change 0.2 volts/0.050 sec
- Vehicle speed is between 5 and 70 mph (8 and 112 km/h)
- Fuel level greater than 15%
- First Air Flow Cell
- Engine RPM 1,000 to 1,300 rpm.
- Engine load 15 to 35%.
- Inferred catalyst temp. 850° F (454° C) to 1,200° F (649° C).
- Number of front O2 switches: 50.
- Second Air Flow Cell
- Engine RPM 1,200 to 1,500 rpm.
- Engine load 20 to 35%.
- Inferred catalyst temp. 900° F (482° C) to 1,250° F (677° C).
- Number of front O2 switches: 70.
- Third Air Flow Cell
- Engine RPM 1,300 to 1,600 rpm.
- Engine load 20 to 40%.
- Inferred catalyst temp. 950° F (510° C) to 1,300° F (704° C).
- Number of front O2 switches: 30.
- The DTCs associated with this test are DTC P0420 (Bank 1 or Y-pipe system) and P0430 (Bank 2). Because an Exponentially Weighted Moving Average algorithm is used for malfunction determination, up to six driving cycles may be required to illuminate the MIL during normal customer driving. If KAM is reset or the battery is disconnected, a malfunction will illuminate the MIL in 2 drive cycles.