Spark plugs, wires, distributor caps and rotors break down and fail over time due to exposure to moisture, oil, grease and under hood temperatures that can reach as much as 550 to 700 degrees. Worn spark plugs require higher voltage, forcing the ignition module and coil to produce increasingly higher and higher voltages to overcome the resistance. This breaks down the spark plug wire's insulation causing the ignition module and coil to produce even more voltage, creating even higher temperatures within them. The increased resistance weakens transistors and electrical insulation within modules, coils and computers, resulting in intermittent and premature failure. For maximum fuel economy and peak engine performance, your spark plugs should be replaced every 30 months or 30,000 miles, unless your vehicle is equipped with 100,000-mile platinum tipped spark plugs. Vehicles 15+ years old are more likely to have standard plugs with approximate life expectancy of 12,000-15,000 miles, If your vehicle has over 60,000 miles and you haven't replaced your spark plug wires, distributor cap and rotor, you may experience a variety of drivability problems that are often difficult and very costly to diagnose and repair.