ASE L1 Advanced Engine Performance Practice Test 2026 – Complete Exam Prep

When carbon monoxide (CO) levels are constant, but there is an increase in oxygen (O2) and a decrease in carbon dioxide (CO2), this can indicate an issue with the exhaust gas recirculation (EGR) system. The role of the EGR system is to recirculate a portion of the engine's exhaust back into the combustion chamber to reduce nitrogen oxide (NOx) emissions and help control combustion temperatures.

If the EGR is leaking, it can allow excess oxygen to enter the combustion chamber. This can lead to incomplete combustion, which is shown by the unchanged CO levels as CO is still being produced, but the increase in O2 indicates that the air-fuel mixture is not optimal. The decrease in CO2 signifies that there is less complete combustion happening, as CO2 is typically a product of complete combustion.

This scenario aligns with the symptoms of a malfunctioning EGR system, where it is not effectively channeling exhaust gases, leading to higher oxygen levels and lower CO2 levels due to changes in how the fuel is being burned.