Integral to the engine’s fuel ignition system, ignition leads are responsible for carrying the electrical current from the ignition coil to the spark plug. Sometimes known as high-tension, or HT, leads (referring to the voltage they carry), each ignition cable encloses a single wire through which the current flows before returning via the opposite battery terminal or negative terminal. There are two types of ignition leads. Older cars have copper-core leads, which are quite stiff, while newer models have carbon-core leads, which are more flexible. The carbon leads come fully assembled (with the distributor, coil connections and plug caps attached), but you will need to assemble the components of the copper leads when you replace them yourself. Common symptoms of faulty ignition leads include bad starting, misfiring, rough running and radio interference. Damage is usually caused by worn-out cable insulation or dirty cable connectors. Left unchecked, this damage could lead to excessive fuel consumption and MOT emissions failures.