Your engine starts due to an air and fuel mixture that gets ignited by a spark of electricity generated by an ignition coil that travels through your spark plug to a cylinder. Spark plugs are one of the most important parts of the ignition system. If they aren’t replaced when needed, your engine can misfire due to the air-fuel mixture being hard to ignite. Your fuel efficiency can also decrease. If you notice either of these signs, get the system looked at and buy replacement spark plugs if needed.