Which symptom may indicate a faulty ignition coil?

A faulty ignition coil primarily affects the ignition system by failing to provide the necessary spark to ignite the air-fuel mixture in the engine's cylinders. When the ignition coil is malfunctioning, it can lead to inconsistent or weak sparks, causing the engine to misfire. Engine misfires occur when one or more cylinders do not fire as they should, leading to performance issues such as rough running, hesitation during acceleration, increased emissions, and overall reduced engine efficiency.

In contrast, while symptoms such as engine overheating, decreased fuel efficiency, or increased battery voltage could be related to various different engine or electrical system issues, they do not directly indicate problems with the ignition coil itself. Engine overheating could be caused by cooling system failures, decreased fuel efficiency might arise from other factors such as fuel injectors or air intake problems, and increased battery voltage may indicate an issue with the charging system. Thus, the best indicator of a faulty ignition coil is indeed engine misfires.