Which factor can adversely affect the outcome of a Smog Check?

A dirty air filter can significantly hinder a vehicle's performance and, consequently, its emissions output. When the air filter is clogged, it restricts airflow to the engine, leading to a rich fuel mixture where excess fuel is burned compared to the air intake. This not only results in poorer fuel efficiency but also increases the number of pollutants emitted from the vehicle's exhaust system. During a Smog Check, vehicles are assessed for their emissions levels, and a dirty air filter can lead to higher readings, potentially causing the vehicle to fail the test.

In contrast, proper tire inflation and recent vehicle maintenance generally support efficient vehicle operation and lower emissions. Using high-quality fuel typically enhances combustion efficiency as well, helping to reduce the levels of harmful exhaust. Thus, while all these factors play important roles in a vehicle's performance, a dirty air filter stands out as a direct cause of increased emissions that can adversely affect the Smog Check outcome.