During an emissions test, excess levels of which gas is typically measured?

During an emissions test, carbon monoxide is typically measured because it is a significant pollutant that can indicate the efficiency of the vehicle's combustion process. Carbon monoxide is a colorless, odorless gas that is produced when fuel is not burned completely in an engine. High levels of carbon monoxide in a vehicle's exhaust can suggest that the engine is running too rich (too much fuel relative to air) or has some malfunction that prevents proper combustion.

The presence of excess carbon monoxide is concerning not only because it is harmful to human health but also because it reflects inefficiencies in fuel usage, which can lead to increased environmental pollution. Consequently, regulations and emissions testing focus on monitoring and reducing carbon monoxide emissions as part of broader efforts to improve air quality.