The Toyota spark plug wires are the electrical conduits that are used to transfer the voltage from either the distributor or the ignition coil to the spark plugs so the controlled explosion of the air and fuel mixture in the combustion chambers can be made possible. It is the manner in which this explosion is initiated that is one of the differences between the internal combustion engine and a diesel engine. With the highly efficient engines Toyota is now producing, the use of carbon composite cores in the spark plug wires is now standard equipment on all models. The carbon composite fibers have the lowest possible resistance in the transfer of an electrical impulse that can be used on a car or truck engine that is available today. This transfer of an electrical impulse by the carbon composites in the spark plug wires is improved from the previously used copper core wires. Just like the older copper core spark plug wires, the ones with carbon fibers also wear out in time. The carbon composite fibers become brittle and are the reason for the increase in resistance of the electrical impulse that travels through them. On average the Toyota spark plug wires will last about 2 to 3 tune-ups (60,000 to 90,000 miles) before they should be replaced. It is all dependant on just how efficient the Toyota owners desire their engine to operate. Even if just one of the wires has gone bad on your Toyota engine, and an increased level of resistance can be detected in it, they all should be replaced. The remaining wires will be in similar conditions as the faulty one. In addition unless you can build your own spark plug wire, they can only be purchased as a set.