Older vehicles with distributor-type ignition systems use ignition (or spark plug) wires to send electrical current from the distributor to individual spark plugs, with the timing based on their firing order. A separate wire runs from the ignition distributor cap to each spark plug — actually thick cables capable of carrying 40,000 volts or more. One end of each wire has a boot designed to fit snugly on the cap, while the other end has a boot that fits onto the spark plug. Ignition wires often need to be replaced as part of an engine “tune-up” because the wires can crack or the electrical contacts can wear out, making it hard to start a car or causing the engine to run roughly or sluggishly. Driving for extended periods with bad ignition wires can also damage catalytic converters
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