Check the listing for details. Inline Spark Plug Tester - Engine Ignition Diagnostic Tool for Cars & Lawnmowers. Condition: New. Listed at 26.59 USD. Designed for ease of use, this tool allows you to troubleshoot all internal combustion engines in just about 30 seconds. Simply connect the tester between the spark plug and the plug wire; if a spark is present, the tester will light up, helping you identify dirty spark plugs, defective points, or bad connections.