If a vehicle failed smog due to high NOx and the catalytic converter is intact, what should be checked next?

When a vehicle fails smog testing due to elevated levels of nitrogen oxides (NOx) and the catalytic converter is confirmed to be intact, the next logical component to check is the exhaust gas recirculation (EGR) system.

The EGR system plays a crucial role in reducing NOx emissions by recirculating a portion of the engine's exhaust gas back into the intake manifold. This process lowers the combustion temperature, as the exhaust gas displaces some of the oxygen and thus decreases the formation of NOx, which is generated at high temperatures. If the EGR system is malfunctioning—be it due to a blockage, a faulty valve, or any other issue—it can lead to increased NOx production by allowing higher combustion temperatures.

In examining other components, while the fuel pressure regulator and oxygen sensors are indeed important for overall engine performance and emissions, they are less directly related to the specific issue of high NOx levels compared to the EGR system. The air filter, although necessary for maintaining good airflow and engine performance, does not have a substantial impact on NOx emissions in the same way that the EGR does. Therefore, focusing on the EGR system following a NOx failure is essential for diagnosing and correcting the