Spark plugs need replacing when a car has trouble starting or accelerating or when the engine misfires, surges or produces a rough sound. Vehicle manufacturers offer recommendations for changing spark plugs that can be between every 30,000 miles and 100,000 miles, depending on the type of plugs.
Spark plugs are devices in the cylinder head that produce an electrical spark that ignites fuel in an internal-combustion engine. They suffer natural wear and accumulated deposits over time that can drain away voltage before it forms a spark. If drivers do not change worn or dirty spark plugs, reduced performance can result.
While every engine misfires occasionally, if the number of incidents increases per mile it can result in a decline in ignition performance that makes it hard to start a car or causes it to buck, stall or run rough. Vehicles produced from 1996 have onboard systems that track ignition misfires. Diagnostic systems illuminate a warning light when the misfires exceed a certain limit, which is an indication that it is time to replace the spark plugs. Even when vehicles are not equipped with these systems, drivers should consult the vehicle manufacturer's recommendation and change the spark plugs as part of a regular preventative maintenance routine before the spark plugs deteriorate to the point that they create problems.