New spark plugs help keep your engine at its peak performance and efficiency levels. Old spark plugs will misfire from time to time. But as those misfires get more and more frequent, they also get more troublesome. Frequent misfires mean increased exhaust emissions, wasted gas and reduced power. Misfires caused by worn spark plugs will cause older vehicles to buck, stall, run rough or start slow. Vehicles made since 1996 have OBD II onboard diagnostic systems that track ignition misfires and will sometimes display a warning light.