Ignition system failures strand drivers and frustrate owners, but diagnosing the problem takes less time than most realize. Spark plug wires and ignition coils serve different functions in your engine's firing system, and distinguishing between failed components saves money and guesswork. Worn wires show up through misfires, rough idle, and poor fuel economy. Failing ignition coils produce similar symptoms but demand different repair strategies. Mechanics check resistance on wires with multimeters and test coil output directly. Modern vehicles with coil-on-plug designs simplify diagnosis compared to older distributors that require more involved troubleshooting. Understanding which component actually failed prevents unnecessary parts replacement and keeps repair bills reasonable. A basic visual inspection often reveals cracked or deteriorated wires before you need professional help. Knowing these distinctions separates competent diagnosis from expensive trial-and-error repairs.