Electricity is conducted via ignition wires between a centre tower of the distributor cap and centre tower of the coil and between the distributor cap and the spark plugs. They could be visually tested through the bending of the set gently to check whether they have cracks, cracks may indicate a need to replace the wires, and replacing the wires in sets is better than replacing the wires separately. Also, it is important to test the resistance using ohmmeter, where 16 KH/m is supposed to be the expected resistance (1000mm).