Which emission pattern indicates a normal functioning EGR system?

A normally functioning Exhaust Gas Recirculation (EGR) system aims to reduce nitrogen oxide (NOx) emissions by recirculating a portion of the engine’s exhaust back into the intake manifold. This process lowers combustion temperatures, which in turn reduces the formation of NOx.

In the context of emission patterns associated with a working EGR system, the ideal outcome would be a notable decrease in NOx levels alongside relatively stable levels of hydrocarbons (HC) and carbon monoxide (CO). Emissions such as HC and CO should not show significant changes because the primary purpose of the EGR system is to mitigate NOx production without adversely affecting the combustion efficiency indicated by these other emissions.

While some options mention changes in HC or CO, these are not typical indicators of effective EGR operation. Therefore, observing stable CO levels suggests that the EGR system is operating correctly, maintaining the balance of combustion without causing secondary emissions issues. Thus, a lack of change in CO when the EGR system is functioning normally confirms that the combustion process remains efficient and effective in controlling NOx emissions.