Catalyst Monitoring

Catalyst Monitor Heated Oxygen Sensor (HO2S 2)

To control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gases, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen and oxygen. The PCM has the ability to monitor this process using the HO2S 1 and the HO2S 2 heated oxygen sensors. The HO2S 1 sensor produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. The HO2S 2 sensor produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalyst's ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the HO2S 1 signal will be far more active than that produced by the HO2S 2 sensor.

In addition to catalyst monitoring, the HO2S 2 heated oxygen sensor has a limited role in controlling fuel delivery. If the HO2S 2 signal indicates a high or low oxygen content for an extended period of time while in closed loop, the PCM will adjust fuel delivery slightly to compensate.

If the oxygen sensor pigtail wiring, connector or terminal are damaged, the entire oxygen sensor assembly must be replaced. DO NOT attempt to repair the wiring, connector or terminals. In order for the sensor to function properly, it must have a clean air reference provided to it. This clean air reference is obtained by way of the oxygen sensor wire(s). Any attempt to repair the wires, connectors or terminals could result in the obstruction of the air reference and degrade oxygen sensor performance.

Catalyst Monitor Diagnostic Operation

The OBD II catalyst monitor diagnostic measures oxygen storage capacity. To do this, heated sensors are installed before and after the Three-Way Catalyst (TWC). Voltage variations between the sensors allow the control module to determine the catalyst emission performance. As a catalyst becomes less effective in promoting chemical reactions, its capacity to store and release oxygen generally degrades. The OBD II catalyst monitor diagnostic is based on an correlation between conversion efficiency and oxygen storage capacity. A good catalyst (e.g. 95% hydrocarbon conversion efficiency) will show a relatively flat output voltage on the post catalyst Heated Oxygen Sensor (HO2S 2). A degraded catalyst (65% hydrocarbon conversion) will show greatly increased activity in output voltage from the post catalyst HO2S.

The post-catalyst HO2S is used to measure the oxygen storage/release capacity of the catalyst. A high oxygen storage capacity indicates a good catalyst; low oxygen storage capacity indicates a failing catalyst. The TWC and HO2S 2 must be at operating temperature to achieve correct oxygen sensor voltages like those shown in the Post-Catalyst HO2S Outputs graphic.

The catalyst monitor diagnostic is sensitive to the following conditions:

^ Exhaust leaks
^ HO2S Contamination
^ Alternate fuels.

Exhaust system leaks may cause the following results

^ Prevent a degraded catalyst from failing the diagnostic.
^ Cause a false failure for a normally functioning catalyst.
^ Prevent the diagnostic from running.

Some of the contaminants that may be encountered are phosphorus, lead, silica, and sulfur. The presence of these contaminants at any HO2S will prevent the TWC diagnostic from functioning properly.