What is a common cause of smog test failure indicated by HC at 300 ppm and CO at 4.5%?

The chosen answer highlights a leaking fuel injector as a common cause of smog test failure, particularly in the context of high hydrocarbon (HC) emissions and carbon monoxide (CO) levels. In the scenario where HC reads at 300 ppm, that indicates incomplete combustion of fuel in the engine. A leaking fuel injector can significantly contribute to this issue as it could cause an excess of fuel to enter the combustion chamber, leading to a rich fuel mixture. This rich condition prevents the complete combustion of the fuel, resulting in elevated HC emissions.

Moreover, the CO level at 4.5% suggests that there is a significant amount of unburned fuel being produced. Carbon monoxide is formed during the incomplete combustion of hydrocarbons, which directly correlates with the action of a fuel injector that is malfunctioning. The presence of these high HC and CO levels in the emissions strongly indicates that a problem with the fuel delivery system, such as leaking fuel injectors, could be the root cause of the smog failure.

In contrast, the other options might contribute to smog issues but do not directly correlate with the specific symptoms of high HC and CO emissions in the same manner as a leaking fuel injector. For instance, a vacuum leak would typically lead to