What might reduced CO₂ levels indicate about a vehicle’s combustion process?

Reduced CO₂ levels in a vehicle’s exhaust can provide significant insights into its combustion process. CO₂ is a product of complete combustion of fuel. Therefore, lower levels of CO₂ can indicate two main issues:

First, poor combustion efficiency occurs when the fuel is not burned effectively. This inefficiency can lead to incomplete combustion, where not all the fuel reacts properly, hence producing lower quantities of CO₂ along with other harmful emissions like carbon monoxide (CO) and unburned hydrocarbons.

Second, excess air in the fuel-air mixture can also result in reduced CO₂ levels. When there is too much air in the mix, it dilutes the combustion process, leading to a situation where there is insufficient fuel to utilize the available oxygen fully. This results in lower CO₂ production, as not all the fuel is combusted completely due to the excess air.

Thus, reduced CO₂ levels can be indicative of both poor combustion efficiency and the presence of excess air in the mixture, making the option that states both conditions correct. A combination of these factors can affect engine performance and emissions, making it vital for technicians to address these issues when diagnosing problems in the combustion process.