The distributor cap, the rotor, and the spark plug wires deliver high-voltage spark to the spark plugs. They are subject to insulation breakdown, corrosion fouling, and electrode wear and damage. Inspection every 30,000 miles (48,000 km) or 2 years, and replacement as necessary, are recommended to ensure maximum ignition system efficiency. Guidelines for visual inspection and testing, and instructions for replacement are found in IGNITION.