Catalyst Efficiency Monitor Steady State

PURPOSE
The Steady State Catalyst Efficiency Monitor is an on-board strategy designed to monitor and determine when a catalytic converter has fallen below the minimum level of effectiveness in its ability to control exhaust emissions.

OPERATION
The monitor relies mainly on Heated Oxygen Sensors (HO2S) located downstream of the catalytic converter to infer catalyst efficiency based on oxygen storage capacity. The oxygen storage capacity of a high efficiency catalyst will have a slower downstream HO2S(s) switching frequency compared to the switching frequency of the upstream HO2S(s). As catalyst efficiency deteriorates, its ability to store oxygen declines causing the downstream HO2S(s) to switch more rapidly approaching the switching frequency of the upstream HO2S(s).

The monitor uses this HO2S switching characteristic to evaluate the catalyst when the monitor is enabled. Input from the Engine Coolant Temperature (ECT), Intake Air Temperature (IAT), Throttle Position (TP), Crankshaft Position (CKP), and Vehicle Speed Sensor(VSS) sensors is required to enable the Catalyst Monitor Also, a calibratable time must have elapsed since engine start up and operation in closed-loop fuel control. Once activated, closed-loop fuel control is temporarily transferred from the upstream HO2S(s) to the downstream HO2S(s). The monitor then analyzes the downstream HO2S signal switching frequency to determine if the catalytic converter has failed.

DTCs and MIL Operation

Catalyst Efficiency Monitor - Steady State:

1 In the Steady State Catalyst Efficiency Monitor test, closed loop fuel control is transferred from the upstream HO2S(s) to the downstream HO2S(s). The switching frequency of the downstream HO2S(s) output is measured. This actual measured output frequency is called the test frequency. The test frequency is an indication of the oxygen storage capacity of the catalytic converter. The slower the test frequency, the higher the efficiency of the catalytic converter.

A second frequency called the calibrated frequency is calculated based on engine rpm and load. The calibrated frequency serves as a high limit threshold for the test frequency. If the test frequency is less than the calibrated frequency, the catalytic converter passes the Catalyst Efficiency Monitor test. If not, the catalytic converter or system is considered to have failed and a Diagnostic Trouble Code (DTC) is stored.

The DTCs associated with this test are DTCs P0420 and P0430.

2 The MIL is activated after a fault is detected on two consecutive drive cycles.