Car Spark Plug Tester Dual Holes Spark Tester. Wide Working For 12V Gasoline Vehicles Ignition Coil Tester With A Protective, Spark Plug Not Included. Place the Spark Plug into tester hole 1 or 2; If you want to compare the new spark plug and old one, place new spark plug into hole 1 and the old on into hole 2.