12v Car Spark Plug Tester. 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. Turn the Key Switch 4 Clockwise to Turn on Tester, Indicator 3 Will Light up, Tester Is Now Working at 1000rpm/Min.
