Switch Ratio Method
- In order to assess catalyst oxygen storage, the monitor counts front and rear HO2S switches during part throttle, close-loop fuel condition after the engine is warmed-up and inferred catalyst temperature is within limits. Front switches are accumulated in up to nine different air mass regions or cells although three air mass regions is typical. Rear switches are counted in a single cell for all air mass regions. When the required number of front switches has accumulated in each cell, the total number of rear switches is divided by the total number of front switches to compute a switch ratio. A switch 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 switch ratio exceeds a calibrated threshold switch 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 Switch Ratio Monitor Entry Conditions: Part throttle with no rapid throttle transients 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) Engine load greater than 10% Time since entering close loop is 30 seconds Vehicle speed is between 5 and 70 mph (8 and 112 km/h) Inferred Catalyst Mid-bed Temperature of 900° F (482° C) Mass air flow is between 1 and 5 lbs/min Fuel level greater than 15% EGR is between 1 and 12%
- 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.