Which symptom might indicate a malfunctioning alternator?

Dimming headlights can indicate a malfunctioning alternator because the alternator is responsible for supplying electrical power to the vehicle's systems when the engine is running. If the alternator is not functioning properly, it may not provide adequate voltage to keep the headlights bright, resulting in a noticeable dimming effect, especially at idle or when other electrical components are in use.

The alternator charges the battery and powers electrical systems such as lights, ignition, and other accessories. When there's a failure or deficiency in the alternator's performance, the electrical load can exceed the alternator's output, leading to symptoms like dimming headlights. This symptom can often be more pronounced at lower engine RPMs when the alternator is generating less power.

Therefore, when headlights dim, particularly when engine performance seems otherwise normal, it presents a strong sign that the alternator may not be working correctly.