NOx is NOT caused by poor combustion. NOx is created when there is an excessive amount of air present during combustion. The oxygen in the air combines with the fuel over a very short time period, releasing all the heat in the fuel at once. The high temperature allows the nitrogen (nearly 80% of air) to combine with any left over oxygen, creating NOx. If there is less air present combustion will occur over a longer time period, releasing the heat more gradually, creating less NOx. If there is less air present there will be fewer O2 molecules left over to combine with the nitrogen.

Incomplete fuel combustion will produce high levels of carbon monoxide and hydrocarbon at the tail pipe, usually caused by a lack of sufficient air to support combustion, or by poor fuel atomization (bad injectors) that interferes with the combustion process.