When diagnosing high NOx emissions, which system should be considered first for issues?

When diagnosing high nitrogen oxides (NOx) emissions, the EGR (Exhaust Gas Recirculation) system should be the primary focus. The EGR system is specifically designed to reduce NOx emissions by recirculating a portion of the engine's exhaust gases back into the intake manifold. This process lowers combustion temperatures, which in turn helps reduce the formation of NOx.

If the EGR system is malfunctioning due to issues such as clogs, leaks, or a faulty EGR valve, it can lead to elevated NOx levels. Therefore, the state and functionality of the EGR system are critical in understanding and addressing high NOx emissions.

While the fuel system, ignition system, and exhaust system can also affect emissions, they do not specifically target the reduction of NOx in the same way that the EGR system does. By prioritizing the EGR system in the diagnostic process, technicians can more effectively identify and resolve issues resulting in excessive NOx emissions.