Three Way Catalyst (TWC) Oxygen Storage Capacity

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).




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.