Which of the following statements is true about the effects of a vehicle's emissions system?

A malfunctioning Three-Way Catalyst (TWC) can indeed increase hydrocarbons (HC) emissions. The TWC plays a crucial role in reducing harmful emissions from the exhaust gases produced by combustion engines. It is designed to convert carbon monoxide (CO), unburned hydrocarbons (HC), and nitrogen oxides (NOx) into less harmful substances before they are released into the environment. When the TWC is not functioning properly—perhaps due to clogging, poisoning, or thermal damage—it is unable to effectively carry out these conversions, leading to higher levels of unburned hydrocarbons being emitted. This is critical for understanding how the emissions system impacts environmental pollution as well as vehicle compliance with emissions regulations.

In regard to the other statements, not all vehicles are equipped with a Positive Crankcase Ventilation (PCV) valve; some older models might not have this system. While a functioning Exhaust Gas Recirculation (EGR) system is intended to reduce NOx emissions, if it were to malfunction, it could potentially lead to increased emissions, contrary to the statement's implication. Lastly, a fuel line leak typically results in increased carbon monoxide emissions rather than a decrease, as unburned fuel can escape into the atmosphere and contribute