What is indicated if CO levels drastically decrease while NOx levels increase?

When carbon monoxide (CO) levels drastically decrease while nitrogen oxides (NOx) levels increase, it typically indicates a lean mixture condition in the engine. A lean mixture occurs when there is an excess of air in relation to the amount of fuel being injected. This excess airflow promotes more complete combustion of the fuel, which results in lower CO emissions since CO is a product of incomplete combustion.

Simultaneously, the increase in NOx levels can be attributed to the higher combustion temperatures that occur in a lean mixture. Higher temperatures favor the formation of nitrogen oxides, as NOx is primarily produced under high-temperature conditions when oxygen is plentiful. Therefore, a simultaneous drop in CO and rise in NOx is a strong indicator of a lean fuel-air mixture.

In contrast, a rich mixture would lead to higher CO emissions and lower NOx levels due to incomplete combustion resulting from the excess fuel. Normal operation would not typically show such a drastic change in emissions, and cylinder misfires would not commonly lead to a consistent drop in CO and rise in NOx levels as described.