Catalyst Efficiency Monitor

DESCRIPTION
The Catalyst Efficiency Monitor is a self-test strategy within the Powertrain Control Module (PCM) that determines when a catalyst has fallen below the minimum level of effectiveness in its ability to control exhaust emissions.

Hardware
The components that make up the Catalyst Efficiency Monitor are the downstream Heated Oxygen Sensor(s) (HO2S) and the catalyst(s).

Operation

Catalyst Detection:

Closed loop fuel control is transferred from the upstream HO2S(s) to the downstream HO2S(s). Calculations are made within the monitor strategy to determine actual output frequency of the downstream HO2S(s). The actual output frequency calculated is called the test frequency. The test frequency is an indication of the oxygen storage capacity of the catalyst. If the test frequency is slow, then the oxygen storage capacity of the catalyst is high.

A second frequency called the calibrated frequency is found using the engine rpm and load. The calibrated frequency serves as a high limit for the test frequency. If the test frequency is less than the calibrated frequency, the catalyst passes the Catalyst Efficiency Monitor test. If not, the catalyst (or a system/component used by the catalyst monitor) is malfunctioning.

DTCs and MIL Operation
When a malfunction has been present for three consecutive drive cycles, the DTC is stored in the PCM and the MIL is turned On. The MIL is turned Off after three consecutive trips without the same malfunction being detected provided no other DTCs are stored that would independently turn the MIL On. The DTC will be erased from memory after 40 warm-up cycles without the malfunction being detected after the MIL is turned Off. Another method of erasing the DTC is initiating a Powertrain Control Module Reset.