The ability of the Subaru spark plug to create an arc of electricity thousands of time a minute is the power behind the explosions that move your vehicle on the road. The flow of this electrical current travels through the spark plug to the ground at the electrode at the other end of the plug. Most Subaru owners are unaware that the center section of their spark plugs is hollow. This is the area covered by the ceramic shell. The electricity enters and then jumps from one end to the other end of the Subaru spark plug. It is this hollow ceramic shell that insulates the two ends of the plugs from each other. If for any reason there is a crack or hole in this shell, the spark plug will fail to function. The typical time the ceramic portion of the plug to become damaged is when it is being handled by the technician replacing them on your Subaru engine. If just one of the plugs comes in contact with a hard surface (e.g. if it was dropped on the floor), this ceramic shell could be damaged causing the plug to become faulty. If one of the spark plugs you have purchased for your tune-up is faulty, then a new replacement is required. Never use just any plug you have laying around in your garage to just fill in for even a short period of time. The plugs you have purchased for your Subaru engine are made to operate in a specific heat range and depth inside of your engine. If either or both of those characteristics are different on the replacement plug, damage could occur to your engine up to and including a hole being created in the top of the piston.