Car Spark Plug Tester Detector with Dual Hole Advantage: Dual Hole Car Spark Plug Tester Detector have two testing holes, you can test two spark plugs meanwhile and and if you suspect any spark plug works improperly you could compare and test to make accurate judgment. Usage: Test spark plug performance and ignition strength Application Range: 12V Gasoline vehicles car spark plugs Car Spark Plug Test principle: In automobile ignition system, the high-voltage ignition coil generates a high-voltage (10000 volts or more), and the spark plug diverts it form high -tension ignition coil into the cylinder engine, spark between the spark plug electrodes will ignite the mixture. The Detector could tell the good spark plugs from the bad by the strength of the spark. Car Spark Plug Tester.