Taking the time to do simple diagnostics can save you large repair bills - after all, why pay a dealership to do these diagnostics when you can order your GMC spark plug wires here and replace them yourself? You can test the resistance level for your GMC vehicle by first checking the factory service manual for your car, where you can find a resistance rating for the spark plug wires. If you have an ohm meter, you can measure the GMC spark plug wires' resistance from end to end, checking with the owner's manual to see if they are within specifications. If a spark plug wire has a deteriorated core the resistance from one end to the other will be very close to infinity. The reading means that there's a virtual separation of the conductor. Spark plug wires for most cars have a resistance of 10,000 to 15,000 ohms per foot of length. Be aware that some after-market replacements may claim to work for a wide range of vehicles, but they may not be a good match for your vehicle.  The core is usually constructed of carbon impregnated fiber or wire; they can be made of anything from nylon to Kevlar. That conducting core of the wire is bent over at the wire end and the boot metal is crimped over the core/insulation of the wire.