What is indicated by no change in CO and an overall decrease in CO2 and a large decrease in NOx?

The situation described indicates a normally functioning Exhaust Gas Recirculation (EGR) system. The EGR system is designed to reduce nitrogen oxides (NOx) emissions by recirculating a portion of the engine’s exhaust back into the intake manifold.

In the case where there is no change in carbon monoxide (CO) and a notable decrease in carbon dioxide (CO2) along with a significant reduction in NOx, it suggests that the EGR system is effectively diluting the combustion mixture with exhaust gases. This dilution lowers combustion temperatures, which in turn reduces the formation of NOx.

The stability of CO levels indicates that the combustion process is not negatively affected and that the air-fuel mixture is still being combusted efficiently. The decreased CO2 might suggest a somewhat leaner combustion state, likely due to the introduction of inert exhaust gases via the EGR system. In this scenario, emissions of NOx are significantly reduced, aligning with the primary operational goal of a properly functioning EGR system.

The other options do not adequately describe the provided scenario; for example, a normally functioning engine or ignition would typically not show such specific changes in emissions data as described, and a properly functioning air injection system would help in reducing CO levels rather