Symptoms of a bad coil pack include rough engine idling, misfiring, lack of power during acceleration, gasoline odor emanating from the exhaust system and illumination of the check engine light. In newer vehicles, the coil pack replaces the distributor.
A coil pack is a collection of ignition coils that transforms power from the automobile's battery to generate sparks to the spark plugs that drive the engine's cylinders. Coil packs provide better horsepower than distributors used in older vehicles.
A bad coil pack can be diagnosed with a visual inspection of the spark plugs, which show charred or blackened deposits on the spark contact points. However, testing with an ohmmeter definitively determines whether the coil pack is defective and needs to be replaced.