Designed for quick and easy diagnosis ignition/engine issues or a fault in the fuel delivery system. Simply connect the tester inline between the spark plug and the plug wire, if a spark is being sent, the tester will mirror the spark and light up. The flashing indicator light clearly identifies potential issues with the ignition system. Testing Method: Turn off the engine and disassemble the spark plug coil. Connect the black leather of the tester to the spark plug. Connect the metal end of the tester to the spark plug coil. Start the engine. The weak ignition wire is intact and the indicator light flashes.