ASE L1 Advanced Engine Performance Practice Test 2025 – Complete Exam Prep

A decrease in carbon monoxide (CO), carbon dioxide (CO2), and nitrogen oxides (NOx) alongside an increase in oxygen (O2) suggests that the combustion process is not operating under its optimal conditions. This pattern is typically indicative of an exhaust leak.

In the case of an exhaust leak, the exhaust gases that are supposed to exit through the exhaust system can escape prematurely. This reduces the concentration of CO, CO2, and NOx in the exhaust because some of the combustion byproducts are escaping before proper emission control processes can occur. Additionally, the presence of unburned oxygen in the exhaust (indicated by increased O2 levels) can signal that the air-fuel mixture was too lean or that the exhaust gas is being diluted due to the leak.

This situation typically contrasts with the other options presented. Low compression would likely lead to incomplete combustion and higher levels of CO and hydrocarbons, but not necessarily result in abnormal O2 levels. Very retarded timing often produces higher CO and hydrocarbons because the fuel may not burn efficiently, again not aligning with an increase in O2. A worn cam lobe could impact valve timing, contributing to performance issues but wouldn’t specifically cause the exact change in emissions referenced.

The combination