What is a negative effect of worn spark plugs on a vehicle?

Worn spark plugs can significantly impact a vehicle's performance, particularly in terms of fuel efficiency and emissions. When spark plugs are worn, they may not generate a strong enough spark to ignite the air-fuel mixture in the combustion chamber effectively. This incomplete combustion can lead to reduced fuel efficiency since the engine may require more fuel to compensate for the lack of proper ignition.

Moreover, when combustion is incomplete, it can produce higher levels of unburned fuel and other byproducts, which increases emissions. This not only strains the vehicle's emissions control systems but also contributes to environmental pollution. Therefore, the correct choice illustrates a direct consequence of worn spark plugs, showing their crucial role in maintaining both efficient fuel use and lower emissions in an internal combustion engine.