(e) Catalyst Monitor HO2S and Diagnostic Operation

(e) Catalyst Monitor HO2S and Diagnostic Operation
In order to control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), the system uses a three-way catalytic converter. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen.

The Control Module has the ability to monitor this process using the HO2S heated oxygen sensors. The HO2S 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 O2S signal will be far more active than that produced by the HO2S sensor.


The OBD II catalyst monitor diagnostic measures oxygen storage capacity. In order to do this, the heated sensors are installed before and after the Three-Way Catalyst (TWC) The 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, the capacity of the catalyst to store and release oxygen generally degrades. The OBD II catalyst monitor diagnostic is based on a correlation between conversion efficiency and oxygen storage capacity.

A good catalyst (e.g. 95% hydrocarbon conversion efficiency) shows a relatively flat output voltage on the post-catalyst Heated Oxygen Sensor (HO2S). A degraded catalyst (65% hydrocarbon conversion) shows a greatly increased activity in output voltage from the post catalyst HO2S.

The post-catalyst HO2S 2 is used to measure the oxygen storage and release capacity of the catalyst. A high oxygen storage capacity indicates a good catalyst; low oxygen storage capacity indicates a failing catalyst. The TWC, HO2S 2, and HO2S 3 must be at operating temperature in order to achieve correct oxygen sensor voltages like those shown in the Post-Catalyst HO2S 3 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 prevents the TWC diagnostic from functioning properly.