An apparatus and method for visually monitoring and testing in situ the operation of an engine ignition system. A gas discharge device (12), typically a neon trigger tube, glow lamp or gas diode, is provided in the electrical circuit connecting each spark plug to its high voltage source so that the gas discharge device is triggered by firing of the spark plug. Defects in the ignition system, such as faulty ignition coil, high voltage leads, or spark plugs, can be visually detected from the failure of the discharge devices to flash, or a degradation in the illumination of the discharge devices. Each gas discharge device (12) can be incorporated into a connector unit (10) between the distributor (20) and a respective high voltage lead (30).