Catalyst Efficiency Monitor

Catalyst Efficiency Monitor

The catalyst efficiency monitor uses the rear heated oxygen sensor (HO2S), after the catalyst, to infer the hydrocarbon (HC) efficiency based on the oxygen storage capacity of the catalyst. During monitor operation the powertrain control module (PCM) calculates the length of the signal while the sensor is switching. Under normal closed-loop fuel conditions, high efficiency catalysts have significant oxygen storage. This makes the switching frequency of the rear HO2S very slow and reduces the amplitude, which provides for a shorter signal length. As the catalyst efficiency deteriorates due to thermal and chemical deterioration, its ability to store oxygen declines. The rear HO2S signal begins to switch more rapidly with increasing amplitude and signal length. The predominant failure mode for high mileage catalysts is chemical deterioration (phosphorus deposits on the front brick of the catalyst), not thermal deterioration.

The catalyst efficiency monitor calculates the rear HO2S signal lengths for 10-20 seconds during part-throttle, closed-loop fuel conditions after the engine is warmed-up, the inferred catalyst temperature is within limits, and fuel tank vapor purge is disabled. The catalyst monitor is enabled for 10-20 seconds per drive cycle. When the catalyst monitor is active, the PCM commands a fixed fuel control routine. During monitor operation the rear HO2S signal lengths are continually calculated. To determine the index ratio, the calculated rear HO2S signal length is then divided by a calibrated signal length, which has compensation for mass air flow. The calibrated signal length is based on the signal length of an HO2S placed after a catalyst without a washcoat. An index ratio near 0.0 indicates high oxygen storage capacity, hence high HC efficiency. An index 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.

Hybrid Hardware and Monitor Operation

1. The hybrid vehicle exhaust system uses two separate HO2S. The front HO2S is a universal HO2S and is the primary fuel control sensor. This sensor is the first HO2S in the exhaust stream and is referred to as the front or stream 1 HO2S. The last HO2S downstream in the exhaust system is used to monitor the catalyst and is referred to as the rear or stream 2 HO2S. For additional HO2S information, REFER to Heated Oxygen Sensor (HO2S) Monitor Heated Oxygen Sensor (HO2S) Monitor.

Typical monitor entry conditions:

- minimum 5 seconds since start-up at 21°C (70°F)
- engine coolant temperature is between 66°C - 110°C (150°F - 230°F)
- intake air temperature is between -7°C - 82°C (20°F - 180°F)
- time since entering close loop is 30 seconds
- inferred rear HO2S sensor temperature, minimum of 427°C (800°F)
- exhaust Gas Recirculation (EGR) is between 0% - 16%
- part throttle, maximum rate of change 0.24 volt/0.05 sec
- vehicle speed is between 56 -129 km/h (35 - 80 mph)
- fuel level is greater than 15%
- air mass is between 11 - 26 g/sec (1.5 - 3.5 lb/min)
- engine speed is between 1,000 - 2,500 RPM
- engine load is between 20 - 60%

2. The DTC associated with this test is DTC P0420. Because an exponentially weighted moving average algorithm is used to determine a concern, up to six driving cycles may be required to illuminate the MIL during normal customer driving. If the KAM is reset or the battery is disconnected, a concern illuminates the MIL in two drive cycles.

Catalyst Monitor Execution

Catalyst monitor execution is once per drive cycle. Typical monitor duration is 10-20 seconds. In order for the catalyst monitor to run, the HO2S monitor must be complete and the EVAP system functional with no stored DTCs. If the catalyst monitor does not complete during a particular drive cycle, the already accumulated switch/signal data is retained in the KAM and is used during the next drive cycle to allow the catalyst monitor a better opportunity to complete.