Adjustable double hole car spark plug tester detector Ignition Plug Analyzer Diagnostic Tool. Dual Plug Car Kit Spark Plug Tester 12V Gasoline Ignition Plug Diagnostic 2024. 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.