In a smog test, if a vehicle's CO reading is 5.8% and HC is 280 ppm, what is the most likely cause of the failure?

The combination of a high carbon monoxide (CO) reading of 5.8% and elevated hydrocarbons (HC) at 280 parts per million (ppm) indicates incomplete combustion of fuel in the engine. This situation is typically caused by an excess of fuel or a lack of proper air intake.

When a fuel pressure regulator is leaking, it can lead to an increase in the fuel delivery to the engine, causing an overly rich fuel mixture. This excess fuel results in both high CO and HC readings, as the engine is unable to efficiently burn all the fuel present. The faulty regulation of fuel pressure presents a compelling explanation for the failure observed in this smog test.

Other factors like faulty ignition coils or bad oxygen sensors could theoretically contribute to emissions failures, but they typically manifest differently. Faulty ignition coils primarily lead to misfires and incomplete combustion, often producing unburned fuels which may elevate HC but would not necessarily continue to raise CO significantly. A worn catalytic converter usually cannot convert CO and HC effectively, leading to elevated emissions as well; however, this failure tends to show more significantly in a more comprehensive emissions profile rather than just these specific high readings.