Circuit Description

The catalyst monitor test is performed once per drive cycle if all test conditions are met. The main enable criteria to enable a catalyst test are, a steady TP sensor reading and calculated temperature of catalytic converter (using ECT, IAT, load and run time) indicating system is fully warmed up. Once the enable criteria are met, 50 samples of the sensor voltage amplitudes are collected. The results of the 50 samples are averages and converted to a millivolt (mV) reading which indicates the efficiency of the converter.

The catalyst monitor test consists of 2 stages. Stage 1 voltage ranges from -555 mV to +550 mV. When this value goes to greater than 0 mV indicating that stage 1 has failed, the stage 2 test will start to be used in determining the converter efficiency. When stage 2 reaches the fail threshold, MIL will be illuminated and DTC will set. Scan tool can display the value for stage 1 test and indicate if it passed or failed.

Whenever DTCs are cleared or the battery has been disconnected, the results of stage 1 will be reset to -555 mV. Stage 1 will fail when the catalyst efficiency drops to less than 75 percent efficient. After stage 1 fails, the 50 sample test results are used by stage 2. When the results of stage 2 are greater than the fail threshold (catalyst efficiency less than 70 percent), MIL will be illuminated and DTC will set.

DTC can be set due to exhaust system leaks between the front and rear O2S or just the rear O2S leaking, deteriorated or contaminated catalyst. A catalyst monitor test will not be performed if a fault exists in the rear or front 02S circuits.