Catalyst Efficiency Monitor: Notes

The catalyst efficiency monitor uses an oxygen sensor before and 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 sensors are switching. Under normal closed-loop fuel conditions, high efficiency catalysts have significant oxygen storage. This makes the switching frequency of the rear heated oxygen sensor (HO2S) very slow and reduces the amplitude, which provides for a shorter signal length. The front HO2S switches more frequently with greater amplitude, which provides for a longer signal length. As the catalyst efficiency deteriorates due to thermal and chemical deterioration, its ability to store oxygen declines. The post-catalyst or downstream HO2S signal begins to switch more rapidly with increasing amplitude and signal length, approaching the switching frequency, amplitude, and signal length of the pre-catalyst or upstream HO2S. The predominant failure mode for high mileage catalysts is chemical deterioration (phosphorus deposits on the front brick of the catalyst), not thermal deterioration.

For the typical HO2S, the catalyst monitor counts the number of front HO2S switches during part throttle, closed loop fuel conditions after the engine is warmed-up and the inferred catalyst temperature is within limits. The number of front switches are accumulated, depending on the calibration, in up to 3 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. 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.

For the universal HO2S, the catalyst 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. The fixed fuel control routine is the same for every vehicle with universal HO2Ss. During monitor operation the rear HO2S signal lengths are continually calculated. The calculated rear HO2S signal length is then divided by a calibrated signal length, which has compensation for mass airflow. 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.

Inputs from engine coolant temperature (ECT) or cylinder head temperature (CHT), intake air temperature (IAT), mass airflow (MAF), crankshaft position (CKP), throttle position (TP), and vehicle speed sensors are required to enable the catalyst efficiency monitor.

Typical Monitor Entry Conditions:

• Minimum 330 seconds since start-up at 21°C (70°F)
• Engine coolant temperature is between 76.6°C - 110°C (170°F - 230°F)
• Intake air temperature is between -7°C - 82°C (20°F - 180°F)
• Time since entering closed-loop is 30 seconds
• Inferred rear HO2S temperature of 482°C (900°F)
• EGR is between 1% and 12%
• Part throttle, maximum rate of change is 0.2 volts/0.050 sec
• Vehicle speed is between 8 and 112 km/h (5 and 70 mph)
• Fuel level is greater than 15%
• First Airflow Cell
  • Engine RPM 1,000 to 1,300 RPM
  • Engine load 15 to 35%
  • Inferred catalyst temperature 454°C - 649°C (850°F - 1,200°F)
  • Number of front HO2S switches is 50
• Second Airflow Cell
  • Engine RPM 1,200 to 1,500 RPM
  • Engine load 20 to 35%
  • Inferred catalyst temperature 482°C - 677°C (900°F - 1,250°F)
  • Number of front HO2S switches is 70
• Third Airflow Cell

• Engine RPM 1,300 to 1,600 RPM
• Engine load 20 to 40%
• Inferred catalyst temperature 510°C - 704°C (950°F - 1,300°F)
• Number of front HO2S switches is 30

The diagnostic trouble codes (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 to determine a concern, up to 6 driving cycles may be required to illuminate the malfunction indicator lamp (MIL) during normal customer driving. If the keep alive memory (KAM) is reset or the battery is disconnected, a concern illuminates the MIL in 2 drive cycles.