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A lean mixture increasing the voltage needed to jump the plug gap, until the ignition system can no longer supply it. A lean charge is harder to ignite, so demand voltage rises — and a coil that is adequate for a correct mixture may not be for a lean one.

The misfire is therefore a symptom of the mixture rather than the ignition, which is the point: replacing coils and plugs changes nothing while the lean condition remains. An intake leak, a failing airflow sensor or low fuel pressure are the usual sources. Check fuel trims before the ignition parts — strongly positive trims alongside a misfire name the mixture, and the ignition system is being asked for more than it was designed to give.