Which factor can affect the lifespan of a spark plug?

The factor that can significantly affect the lifespan of a spark plug is fuel quality. High-quality fuel contributes to optimal combustion in the engine, which helps maintain the spark plug's condition by preventing excessive carbon buildup, pre-ignition, and fouling. Poor fuel quality, on the other hand, can lead to deposits forming on the spark plug, increased wear, and ultimately a reduced lifespan.

Other factors—such as installation angle, color of the spark plug, and brand—can have some influence on performance, but they do not have the same direct impact on lifespan as fuel quality does. For example, while improper installation can affect spark plug function, it is the quality of the fuel burned that predominantly determines how well the spark plug performs over time.