If the O₂ reading after the catalytic converter is high, what does this suggest?

A high O₂ reading after the catalytic converter indicates that the catalyst may not be using oxygen effectively. In a properly functioning catalytic converter, one would expect the O₂ levels to be low because the converter is designed to oxidize carbon monoxide (CO) and hydrocarbons (HC) and reduce nitrogen oxides (NOx). A high O₂ reading suggests that the exhaust gases have a higher concentration of oxygen than expected, which can be attributed to the catalyst's inability to efficiently convert exhaust gases, indicating a possible malfunction or inefficiency in the catalyst.

This situation typically points to issues such as catalyst damage, overheating, or improper combustion occurring prior to the catalytic converter, leading to insufficient conversion of pollutants and a higher oxygen level remaining in the exhaust gases. Therefore, a high O₂ reading serves as a diagnostic clue that further investigation into the catalytic converter's condition is warranted.