Spark Plug Tester in-Line for Automotive Car Lawnmower Ignition System Coil Engine Coil Tester Diagnostic Tool 6-12 Volt (Pack of 2) Must-have diagnostic tool for any engine with a spark plug Simple to use - just remove rubber boot from spark plug, insert tester & place tester boot on plug Turn the engine over - if ignition is being sent to the cylinder, light in tester glows Package includes 2 pc testers Great time and frustration-saver with cars, motorcycles, boats, chainsaws, lawn mowers & more