What is indicated by a large decrease in NOx when CO levels are decreasing?

A significant decrease in NOx (nitrogen oxides) emissions alongside a decrease in CO (carbon monoxide) levels suggests that the combustion process is becoming more efficient and complete. The most likely cause for this scenario is the adjustment of ignition timing.

When timing is retarded, it often leads to a cooler combustion chamber temperature due to the fuel-air mixture igniting later in the power stroke. This cooler temperature tends to reduce the formation of NOx, which is produced at high temperatures. At the same time, if CO levels decrease, it indicates that there is better combustion efficiency, meaning that a higher percentage of the fuel is being burnt completely, resulting in lower CO emissions.

Therefore, the observed decrease in both NOx and CO together indicates that the combustion process is benefitting from retarded timing, leading to lower temperatures, further reducing NOx formation, while producing less unburned fuel, thereby lowering CO levels as well.