28 - DTC-Based Diagnostics / MODULE, Powertrain Control (PCM) / Diagnosis and Testing

P0420-00-CATALYST EFFICIENCY (BANK 1)


Theory of Operation

The State of Change (SOC) catalyst monitor uses the signals from both the upstream and downstream O2 Sensors to detect aging of the catalyst. Based on the fact that when a catalyst ages, it loses some of its Oxygen Storage Capacity (OSC). As a result, part of the untreated exhaust gases can breakthrough the catalyst and causes the downstream O2 Sensor to deviate from its neutral (Stoichiometric) position. By observing the activities in the downstream O2 Sensor signal, located in the exhaust path behind the Catalytic Converter, the degradation level of catalyst can be detected. In general, the higher the downstream O2 Sensor SOC value, the more exhaust gas breakthrough and the lower the Oxygen Storage Capacity of the Catalytic Converter. The Downstream O2 Sensor response monitor is intended to diagnose a Downstream O2 Sensor that is not moving or stuck in a voltage window and to insure accurate information for catalyst monitor diagnosis.


When Monitored and Set Conditions

When Monitored: This diagnostic runs when the following conditions are met:

•No other O2 Sensor circuit faults active.
•There are several different catalyst efficiency monitoring windows. Each specific window has several different enable conditions. The best opportunity for the catalyst efficiency monitor to run is:
◦Engine temperature above 70°C (158°F).
◦Engine speed between approximately 1400 and 2300 rpm.
◦Vehicle speed between 20 mph and 85 mph, under moderate engine load.


Set Conditions:

•The Powertrain Control Module (PCM) detects a failed catalyst if the final Downstream O2 Sensor state of change test ratio is within the calibrated fail threshold.


Default Actions:

•MIL light will illuminate.


Possible Causes

O2 SENSOR 1/1 RETURN CIRCUIT SHORTED TO GROUND
O2 SENSOR 1/2 RETURN CIRCUIT SHORTED TO GROUND
O2 SENSOR 1/1 RETURN CIRCUIT OPEN
O2 SENSOR 1/2 RETURN CIRCUIT OPEN
EXHAUST LEAK
ENGINE MECHANICAL CONDITION
AGING O2 SENSOR
CATALYTIC CONVERTER