Catalyst Monitor Diagnostic Tests

DESCRIPTION
The emissions control system used on this vehicle has a Three-Way Catalytic (TWC) converter in order to reduce exhaust emissions of Hydrocarbons (HO), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx). The catalyst material that is inside of the converter promotes a chemical reaction in order to combine oxygen with the HC and 00, converting them to water vapor (H20) and harmless Carbon Dioxide (C02). The NOx is converted to Nitrogen (N) by the converter.

The control module has the ability to monitor the efficiency of the catalytic converter through the use of a Heated Oxygen Sensor (HO2S) mounted in the engine exhaust gas stream after the catalytic converter (post-catalyst).

Three-Way Catalyst Oxygen Storage Capacity:

GOOD CATALYTIC CONVERTER
The control module monitors the post catalyst HO2S signal voltage in order to measure the oxygen storage capacity of the converter. A good catalytic converter is indicated when the post-catalyst HO2S signal voltage is relatively steady (2) compared to the pre-catalyst HO2S signal voltage (1).




MALFUNCTIONING CATALYTIC CONVERTER
A malfunctioning catalytic converter is indicated when the post-catalyst HO2S signal voltage is more active (2), similar to the pre-catalyst HO2S signal voltage (1).

In addition to catalyst monitoring the post catalyst HO2S has a limited role in fuel control.

IMPORTANT: Do not use post-catalyst HO2S voltage activity in order to determine the efficiency of the TWC converter unless directed to do so by a diagnostic procedure.

The catalyst monitor on-board diagnostic tests can be effected by the following:

^ A slightly degraded or marginal catalyst
^ Fuel containing high amounts of the following contaminants:
- Sulfur
- Silica
- Lead
- Phosphorus
^ Exhaust system leaks may cause the following
- Prevent a degraded or marginal TWC converter from failing a diagnostic test.
- Cause a normally functioning TWC converter to fail a diagnostic test.
- Prevent the diagnostic test from running.
^ An aftermarket HO2S.
^ An aftermarket catalytic converter.