What is the most probable cause for smog failure in a vehicle with HC at 180 ppm and CO at 3.8%?

In the context of emissions testing, hydrocarbon (HC) levels and carbon monoxide (CO) percentages help diagnose the health of a vehicle's combustion system. When a vehicle shows high HC emissions alongside elevated CO levels, it indicates incomplete combustion, often driven by issues that affect the air-fuel mixture.

A faulty fuel pressure regulator can cause an excessively rich fuel mixture, leading to incomplete combustion and thus higher HC emissions. The high CO level, combined with the elevated HC, suggests that the vehicle is burning too much fuel and creating more unburned hydrocarbons due to that richness. The fuel pressure regulator controls the amount of fuel delivered to the engine, and if it malfunctions, it may allow too much fuel into the combustion chamber, leading to the observed smog failure.

Understanding the other options provides further context. A vacuum leak typically leads to a lean mixture, resulting in lower HC levels and higher CO, which doesn’t match the scenario presented. A leaking fuel injector might contribute to elevated HC but would not typically explain high CO levels in the same way as a faulty regulator. Lastly, a stuck closed PCV valve relates more to crankcase pressures and can influence emissions, but it is not the primary factor for such elevated HC and CO readings as