What does a malfunctioning spark plug typically result in?

A malfunctioning spark plug often results in increased exhaust emissions due to incomplete combustion of the air-fuel mixture in the engine's cylinders. Spark plugs play a crucial role in igniting the fuel-air mixture at the right time, so when they fail or are worn out, they can lead to misfires or inefficient combustion. This inefficiency contributes to higher levels of unburned fuel entering the exhaust system, thereby increasing harmful emissions like hydrocarbons and carbon monoxide.

In contrast, higher fuel efficiency and better ignition timing are outcomes associated with properly functioning spark plugs, as they ensure that combustion occurs optimally. Additionally, a malfunctioning spark plug often leads to rough or inconsistent engine operation rather than quieter functioning, further confirming why increased exhaust emissions is the most accurate consequence of a spark plug issue.