What is a common reason for a vehicle to fail emissions testing?

High levels of nitrogen oxides (NOx) are a common reason for a vehicle to fail emissions testing because NOx is a significant pollutant that contributes to smog and acid rain. During the combustion process, nitrogen and oxygen in the air can react at high temperatures to form NOx compounds. Vehicles equipped with catalytic converters and exhaust gas recirculation systems are designed to reduce NOx emissions, but if these systems are malfunctioning or if the engine is not operating efficiently, NOx levels can rise above acceptable limits.

In emissions testing, the focus is primarily on the pollutants that contribute to air quality issues. While excessive tire wear, low fuel efficiency, and faulty air conditioning might indicate various mechanical issues, they do not directly correlate to emissions pollutants like NOx. Therefore, the presence of high NOx levels is a clear violation of emissions standards and a primary factor in the failure of emissions testing.