In a vehicle that fails a smog test with HC at 220 ppm and CO at 3.2%, what is the most likely cause of the failure?

The most likely cause of a smog test failure characterized by high hydrocarbon (HC) levels at 220 ppm and elevated carbon monoxide (CO) levels at 3.2% is linked to a faulty mass air flow sensor.

When the mass air flow sensor malfunctions, it does not accurately measure the amount of air entering the engine. This can lead to an improper air-fuel mixture, which is either too rich (more fuel than air) or too lean (more air than fuel). In this situation, a rich mixture is more likely, as it can cause incomplete combustion, resulting in excessively high HC emissions.

Hydrocarbons are the unburned fuel particles that are emitted when combustion is incomplete. A rich fuel condition typically causes higher levels of HC because not all fuel is fully combusted, leading to higher readings during emissions testing. Furthermore, the presence of increased CO levels also supports the idea of incomplete combustion, as CO is a byproduct of burning fossil fuels that occurs when there is not enough oxygen to fully oxidize the hydrocarbons.

In summary, a faulty mass air flow sensor can lead to an improper air-fuel mixture and incomplete combustion, which collectively result in both high HC and CO emissions, thereby causing