What symptom may indicate a malfunctioning EGR system?

A malfunctioning Exhaust Gas Recirculation (EGR) system is often indicated by high levels of nitrogen oxides (NOx) emissions. The primary function of the EGR system is to reduce NOx emissions by recirculating a portion of the exhaust gases back into the engine's intake manifold. This process lowers the combustion temperature and reduces the formation of NOx, which is produced at high temperatures.

When the EGR system is not functioning properly, it may either fail to recirculate enough exhaust gas or may not function at all. This can lead to increased combustion temperatures, resulting in elevated NOx emissions. Therefore, the presence of high NOx emissions is a direct symptom of a malfunctioning EGR system, making it the correct choice.

The other options do not directly relate to the EGR system’s malfunction in the same way. Excessive levels of CO2 could be indicative of different combustion issues or rich fuel mixtures rather than directly related to EGR performance. Increased fuel efficiency would be an unlikely symptom of an EGR malfunction as excessive NOx generally correlates with poorer fuel performance. Moreover, low idle RPM could be caused by several factors, including issues with the air-fuel mixture or ignition but does not specifically indicate