What is typically indicated by a NOx reading of 2400 ppm in a smog test?

A NOx reading of 2400 ppm during a smog test indicates a significant emission of nitrogen oxides, which suggests that there is an issue with the vehicle's emissions control systems. In this context, the reading is much higher than acceptable levels, which can indicate a malfunction. One of the primary causes of elevated NOx emissions is clogged EGR (Exhaust Gas Recirculation) passages.

The EGR system is designed to recirculate a portion of the exhaust gas back into the intake manifold, which helps to reduce combustion temperatures and therefore lowers NOx emissions. When the passages are clogged, the EGR system cannot operate effectively, resulting in higher combustion temperatures and an increase in NOx production. This scenario directly correlates with the high NOx reading observed in the smog test.

The other choices do relate to potential issues with the vehicle but do not directly lead to such a high level of NOx emissions. For example, normal engine operation would not result in such elevated readings, while excessive fuel vaporization and defective fuel injectors typically relate more to hydrocarbon emissions rather than NOx, making them less likely sources of the elevated reading observed.