A powerful spark is needed to start up an engine. The right tools to deliver and create this spark are likewise important, such as the spark plug and spark plug wires. Connected to the spark plugs and distributor or ignition coils, the spark plug wires are responsible for delivering the volts to the spark plugs when the engine starts. They are made to be durable to endure the thousands of volts flowing through them. Spark plug wires are also designed to have a lot of resistance to reduce the radio static being created by the ignition system. Normally, a spark plug wire has a resistance of about 10,000 to 15,000 ohms per foot. Anything higher than this can result to malfunction.

Despite their durability, spark plug wires cannot last forever since the insulation can become brittle and the core may develop some cracks. These can damage the spark plug wires and may end up with the electricity flowing elsewhere, leading to engine misfiring, fouled spark plugs, and rough running. The common problem spark plug wires encounter is a break in the insulation. The insulation keeps the electricity in its proper place. If this breaks, the spark can cause some serious damage to the vehicle, often affecting gas mileage. It can also cause unburned fuel to pass into the exhaust system where it can harm the catalytic converter.

One way to prevent problems is to replace the spark plug wires every 30,000 miles (50,000 km). Another is keeping the spark plug wires clean to prevent the possible accumulation of a thin film around the boots at the ends of the wires. Once exposed to moisture, this dirt can create a conductive path to the high voltage, which can drain away the current that is supposed to go to the spark plug. Such maintenance can keep both the vehicle and the engine last longer and perform better.