If emissions indicate a lean mixture with an increase in O2, what could be the underlying cause?

In this scenario, where emissions show a lean mixture alongside an increase in oxygen (O2), the most plausible underlying cause would be an issue with the fuel mixture. A rich mixture, which has an excess of fuel relative to the amount of air, should ideally result in lower oxygen levels in the exhaust because unburned fuel is present. However, if we observe that there is still an increase in O2 levels in the emissions, it could indicate that the fuel is not mixing well with the air or is being burned inefficiently, leading to excess oxygen.

This situation suggests that there is not enough fuel being delivered or utilized properly during combustion, allowing for more unreacted oxygen to pass through the exhaust system, which is indicative of a lean condition. Therefore, the increase in oxygen would not be due to a rich mixture, but rather a misreading or malfunction in how the mixture is being managed.

Understanding this dynamic is crucial for diagnosing engine performance issues, especially those related to emissions. A rich mixture generally results in a higher level of unburned hydrocarbons and lower oxygen, while a lean mixture corresponds with higher oxygen levels in the exhaust, confirming that a conflict in combustion efficiency is present.