What is the most likely cause of a vehicle failing a smog test with HC at 250 ppm, CO at 0.3%, and NOx at 2200 ppm?

In this scenario, a vehicle has failed a smog test with high readings for hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). The most pertinent assessment is the high NOx level, which is significantly above acceptable limits.

A stuck closed Positive Crankcase Ventilation (PCV) valve is a likely cause since it can lead to increased crankcase pressure, which affects the engine's combustion process. When the PCV valve is not functioning properly, it can result in poor airflow and, consequently, incomplete combustion, leading to higher HC emissions. Moreover, excess pressure in the crankcase can cause more blow-by gases, entering the intake manifold and resulting in increased NOx emissions due to elevated combustion temperatures.

High NOx emissions specifically indicate a lean air-fuel mixture or high combustion temperatures, which are often exacerbated when a PCV valve is stuck closed. This malfunction can restrict the proper recirculation of gases and disrupt the overall balance needed for efficient combustion.

While the other potential causes can contribute to emissions issues, they do not explain the high NOx readings as directly as a malfunctioning PCV valve does. A faulty mass air flow sensor may lead to poor air-fuel mixture ratios but does not