Catalyst Efficiency Monitor: Notes

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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. 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. As the catalyst efficiency deteriorates due to thermal and chemical deterioration, the catalyst ability to store oxygen declines. The post catalyst or downstream 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) and thermal deterioration.

The catalyst monitor calculates the rear HO2S signal lengths for 10 to 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 to 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. 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 and high HC efficiency. An index ratio near 1.0 indicates low oxygen storage capacity and 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) (if equipped), 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 universal 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 universal 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 universal HO2S switches is 30

Six drive cycles may be required to illuminate the malfunction indicator lamp (MIL) during normal customer driving, because an exponentially weighted moving average algorithm is used to determine a concern. If the keep alive memory (KAM) is reset, a concern illuminates the MIL in 2 drive cycles.