A lean fuel mixture can lead to what smog test result?

A lean fuel mixture means there is a higher ratio of air to fuel in the combustion process. When an engine runs lean, it typically leads to higher combustion temperatures. These elevated temperatures can facilitate the formation of nitrogen oxides (NOx) as a byproduct of combustion.

NOx emissions increase because high temperatures cause nitrogen in the air to react with oxygen. Consequently, when a vehicle with a lean fuel mixture is subjected to a smog test, it is likely to exhibit high levels of NOx emissions due to this chemical reaction occurring more readily at elevated temperatures.

In contrast, low carbon monoxide (CO) levels are generally associated with a lean mixture as well, but that doesn't explain the formation of NOx, making it less relevant to the question. Similarly, while high hydrocarbon (HC) levels would be more indicative of an overly rich mixture, stable carbon dioxide (CO₂) levels do not directly connect to mixtures that are too lean. Thus, the increase in NOx levels stands out as the most accurate consequence of a lean fuel mixture during smog testing.