That's because spark plug wires aren't actually made of wire. They're made of delicate carbon fibers. Over time, the carbon breaks down and the fibers separate, causing high electrical resistance. That's why it pays to replace your spark plug wires before they wear out.
How often do you need to change your spark plugs?
Vehicle manufacturers make much of the fact that their products come equipped with extended-life spark plugs that can maintain a precise gap for 100,000 miles. However, before you wait that long to replace them, you should bear in mind that spark plugs that hit the 80,000-mile marker are four-fifths worn.
Do I have to change my spark plug wires?
Most of the time, your wires should last well beyond the 30,000 miles your copper spark plugs are rated for. However, there are other factors that can play a role in the timing. Increasing Resistance: Perhaps the best way to tell if your spark plug wires need to be replaced is to test their resistance.