Faulty spark plugs can cause an engine to misfire, but the wires that deliver electricity to the plugs can cause problems as well. It is important for drivers to keep an eye on the wires and routinely inspect them for damage. In order to avoid issues, a good rule of thumb can be to replace the wires when the spark plugs are changed.

If the wires are damaged, the electricity cannot travel to the spark plugs to ignite the fuel in the combustion chamber. The most common cause of faulty wires is a break in the insulation. The rubber encasement is designed to prevent the electricity from escaping before it reaches its destination. When the insulation is cracked or missing, the electricity can arc, or jump, from the wire to any nearby metal parts, which will lead to either a weak spark or nothing at all for the cylinder with the faulty wire. This can cause the engine to run rough and have an adverse effect on gas mileage. When the wires cannot transmit the electricity to the cylinder, the fuel will not be ignited, and it can get into the exhaust system. This can cause damage to the catalytic converter.

Drivers should remember to inspect the wires every now and then to make sure they are in good condition. Catching a faulty wire early can prevent all sorts of damage to your engine and exhaust system. A good time to inspect them is when the spark plugs need to be changed. If that has been done recently, it can be a good idea to pop the hood and check the wires anyway.

Make sure the vehicle is off and inspect the wires, starting at the distributor end and working your way to the spark plugs. First, examine the boots at the distributor end to see if they are torn or broken. Then, inspect the wires themselves for visible damage and bend them gently to see if any cracks appear. It is important to pay attention to how the wires feel when they are bent - they should be pliable and smooth. The final step in the inspection is to remove the plugs, one at a time, and see if there is any problem at the end of the wires. Burns or dark coloration can indicate a problem.

If even one wire has signs of damage, every spark plug wire should be replaced. When one wire goes bad, chances are that the others will soon follow suit. Many newer vehicles use coil on plug ignition systems instead of systems with wires, and these will also sometimes need to be replaced.