Which of the following contributes to excessive nitrogen oxide (NOx) emissions?

The choice identifying a defective EGR (Exhaust Gas Recirculation) valve as contributing to excessive nitrogen oxide (NOx) emissions is correct because the EGR system is designed to reduce the amount of NOx produced during combustion. The EGR valve recirculates a portion of the exhaust gases back into the engine's intake manifold. This reduces the peak combustion temperature, thereby lowering NOx formation. When the EGR valve is defective, it may not function properly, leading to a decrease in exhaust gas recirculation. As a result, the combustion temperatures can rise, increasing the formation of NOx emissions.

In contrast, high fuel pressures can affect engine performance and efficiency, but they do not directly correlate to NOx emissions increase. Frequent oil changes are generally beneficial for engine health and do not contribute to increased NOx emissions. Similarly, cold engine temperatures typically lead to higher emissions of other pollutants during startup, but they do not directly result in excessive NOx. It's the malfunction of the EGR valve that plays a crucial role in managing NOx emissions, making it the relevant factor in this context.