What is the implication of a vehicle having low levels of O₂ during a smog test?

Having low levels of O₂ during a smog test indicates that there is an excess of fuel in the combustion chamber. In a properly functioning engine, oxygen is essential for the combustion process; it combines with fuel to produce energy. However, if the O₂ levels are low, it typically means that either the fuel-air mixture is overly rich or that there isn’t enough air for the fuel to fully combust.

When there’s too much fuel relative to the amount of air, incomplete combustion occurs, leading to higher emissions of hydrocarbons and carbon monoxide, which are both significant contributors to smog and air pollution. This situation points to potential issues such as a malfunctioning fuel injection system, a clogged air filter, or problems with the engine's overall air intake, which would result in inefficient combustion processes. Thus, the presence of low O₂ levels serves as a critical indicator of excess fuel issues during the smog testing process.