If you engine was running great ,good gas milage and smooth accelaration , remove one check and see if the color of the burn in the spark plug is the color of Tan ,if it is check to see if its an NGK and replace it with the same. Remember Japoneese Engines only Use NGK plugs ,but then again all Germain Cars sparks plugs are supplied by NGK. Think why is it. NGK Best technology..... Keep it simple Keep it NGK........