Spark Plug Tester 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. adjustable double hole car spark plug tester Ignition Plug Analyzer Diagnostic Tool Usage: Test spark plug performance and ignition strength Application Range: 12V Gasoline vehicles car spark plugs Widely Application: 100V-240V input voltage . 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 JF109 could tell the good spark plugs from the bad by the strength of the spark. Spark Plug Tester Using Note: 1. Please Turn Off Power before you place spark plug 2. The Spark Plug Tester is a high-voltage so do not touch spark plug with hand at work to avoid danger. Spark Plug Tester User Manual: 1.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 2.Connect to power supply 3.Turn 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 6000rpm/min.The higher the frequency, the stronger the spark strength. Packing List 1 x Spark Plug Tester Main Unit 1 x Power supply connector 1 x Gasket 1 x User manual