What do the spark plug wires do? The spark plug wire has a seemingly simple job: Carry the high-voltage electricity produced by the ignition coil to the terminal of the spark plug. Once at the plu? the electricity travels to the other end of the plu? and jumps a gap between electrodes to produce the spark that ignites the fuel mixture. Because high-voltage electricity looks for an easier path to jump across than a pair of electrodes in a spark plu? containing the electricity within the wire takes a thick jacket of insulatio? and some plugs have an outer jacket to resist cut? high underhood temperature? etc. If that outer jacket is damage? the electricity may leak out to follow the easier path. How do you know you need your spark plug wires replaced? Engine light is on Engine is misfiring Vehicle is running rough Issues starting the vehicle Poor fuel economy Wires are arking How is the spark plug wires replaced by a mobile mechanic? Battery power is disconnected Compression tested (if required) Ignition wires are removed New spark plug wires are installed Battery is connected and system testing How important is having your spark plugs wires replaced? Your spark plugs ignite the air fuel mixture entering the engine. If they are not running at their maximum level it will affect the vehicles operation. When replacing the spark plugs with your tune u? we recommend replacing your wiring harness also known as the ignition wires which carries the voltage to the spark plugs. When changing the spark plug wire? if the spark plugs have not been changed its recommended to have them changed as well at the same time.