What is a sign of a bad ignition coil?

A misfire in the engine is a key sign of a bad ignition coil because the ignition coil is responsible for converting the battery's low voltage to the thousands of volts needed to ignite the air-fuel mixture in the engine's cylinders. If the ignition coil is failing or has failed, it may not provide enough voltage or may cause an inconsistent spark. This can lead to incomplete combustion, resulting in a misfire where the engine fails to fire properly in one or more cylinders. Misfires may manifest as rough idling, a loss of power, or noticeable vibrations, indicating that the ignition system is not functioning as it should.

In contrast, stable idle, excessive engine noise, and high fuel consumption are more indicative of other issues in the engine or vehicle systems, such as fuel delivery problems, more general mechanical issues, or sensors malfunctioning. While these symptoms may occur alongside a misfire, they do not specifically indicate a problem with the ignition coil as directly as a misfire does.