Use to quickly trouble shoot ignition system. Device is �hands-free�. Connects in-line between spark plug and spark plug wire. Shows a duplicate view of the ignition spark. Technician can visually check condition of ignition system at each cylinder. Frequency and intensity of flash indicates whether there is an ignition system problem or a fault in the fuel delivery system. Flash can be seen in bright sunlight. Clear plastic body can withstand rough use and resists automotive chemicals. Protected by mechanical strain relief. Replacement Bulb Part No. 23904.