Over time, the resistance of your Infiniti spark plug wire may wear down.  The insulation may begin to fall apart, causing cracks, leaking energy to the engine body instead of sending it to the spark plug; where it's supposed to go.  Your connections may corrode or vibration may break the wire inside the insulation.  If even one of these problems occurs, you're going to have to replace your spark plug wire.  Three common signs that your vehicle may show if it's time for a new spark plug wire are: miss-firing, hard starting, and poor fuel economy.  You may only experience one of these signals or all three.  Either way, the end result is the same; you need new spark plug wires.  Help your spark plug wires to last as long as possible by keeping them clean.  If you don't check on your spark plug wire once and a while, a thin film can develop outside the plug wire creating a conductive path that will drain the current that should be going to the spark plug.