Just like any other part of your vehicle, your Kia spark plug wire will eventually need to be replaced.  Usually, the resistance in the wire is the first thing to go.  It's important that you replace resistant wires with the same type of wire.  Substituting non-resistant wires for resistant ones will damage your ignition system.  Another reason to replace your spark plug wire is if the insulation that's wrapped around the wire gets brittle and cracks.  Once you've got a crack in the insulation, the energy can leak out to the engine body instead of being sent to the spark plug.  Corrosion of the connectors is another reason to replace your spark plug wire.  The constant vibration of a spark plug wire can lead it to break inside its insulation.  Even if you never look at your spark plug wire, you may know when it's time for a replacement.  A few signs that your spark plug wire needs to be replaced are: miss-firing, hard starting, and decreased fuel efficiency.  Your vehicle may send you one, two, or all three of these signals.  If you're unsure of the status of your spark plug wire, have a professional assess its condition.