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 strengthApplication Range: 12V Gasoline vehicles car spark plugsCar 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 Using Note:1. Please Turn Off before you place spark plug2. The Spark Plug Tester is a high-voltage detector, so do not touch spark plug with hand at work to avoid danger.Car Spark Plug Tester User Manual:1Place 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 22Connect to supply3Turn the key switch 4 clockwise to turn on tester, indicator 3 will light up, tester is now working at 1000rpm/min.4. Again turn the key switch 4 clockwise,you can adjust the working frequency between 1000rpm to 5000rpm/min.The higher the frequency, the stronger the spark strength.Main Unit Weight:220G( Not including Adapter) Adapter: Input 100-240V AC / Output 12V 1A DCColour: black grayMaterial: plasticPackage Contents:1 * Car Spark Plug Tester1 * US 1 * EU Only the above package content, other products are not included.Note: Light and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.