Catalyst Monitor Diagnostic Operation

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.

Figure 7 - Catalyst Monitor HO2S Outputs (Good TWC):

A good catalyst (e.g. 95% hydrocarbon conversion efficiency) will show a relatively flat output voltage on the post-catalyst Heated Oxygen Sensor (HO2S).

Figure 7 - Catalyst Monitor HO2S Outputs (Bad TWC):

A degraded catalyst (65% hydrocarbon conversion) will show greatly increased activity in output voltage from the post catalyst HO2S.
The post-catalyst HO2S 2 is used to measure the oxygen storage/release capacity of the catalyst. Refer to the Post-Catalyst HO2S 2 Outputs graphic). A high oxygen storage capacity indicates a good catalyst; low oxygen storage capacity indicates a failing catalyst. The TWC, HO2S 2 must be at operating temperature to achieve correct oxygen sensor voltages.
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 will prevent the TWC diagnostic from functioning properly.