What can be concluded if a vehicle's exhaust emissions test shows normal CO2 levels and high O2 levels?

When a vehicle's exhaust emissions test indicates normal levels of carbon dioxide (CO2) alongside high levels of oxygen (O2), it suggests that the engine is running lean. A lean mixture means there is more air in the fuel-to-air ratio than is optimal for combustion, leading to incomplete combustion. In a properly functioning internal combustion engine, CO2 is produced when fuel is burned; however, a lean condition can result in excess oxygen remaining in the exhaust, which is observed as high O2 levels.

This scenario indicates that while some fuel is being combusted adequately, there isn't enough fuel relative to the air, which limits the production of CO2. This combination of normal CO2 and elevated O2 levels is a strong indicator of a lean running condition rather than an engine running rich, which would typically show high CO2 and low O2 levels due to excess fuel in the mixture.

Additionally, a restricted exhaust system could lead to symptoms that are not mirrored in the provided emissions numbers, and a combustion process deemed "normal" would likely show balanced readings of CO2 and O2. Thus, the data presented aligns well with the conclusion that the engine is running lean.