Spark plugs, which screw into the top of an engine’s cylinders, create the electric spark required to ignite the compressed air-fuel mixture in gas-powered engines. The right type of plug and the size of the gap between the electrodes at the tip are specified by the automaker, and using the right ones are crucial to proper performance. A fouled, worn or loose plug can cause rough running or abnormal noises. Longer replacement intervals, typically around 100,000 miles in newer cars, have simplified plug maintenance, but they’ve also led manufacturers to justify making them more labor-intensive and expensive than ever to replace.
