Ignition Spark Tester
A diagnostic tool used to test the output and strength of an ignition coil or spark plug wire under load.
An ignition spark tester is a diagnostic tool used to safely verify if an ignition coil or spark plug wire is producing a strong electrical spark. The tester is connected between the spark plug boot (or coil output) and a solid engine ground. It contains a visible gap through which the spark must jump when the engine is cranked.
Using a spark tester is far safer than holding a loose spark plug against the engine block, which can cause electrical shock, damage the ECU, or ignite fuel vapors in the engine bay. Professional testers feature an adjustable gap, allowing the technician to increase the distance the spark must travel. This simulates the high-pressure environment inside the combustion chamber, testing the coil's ability to produce the high voltage (typically 20,000 to 45,000 volts) needed to fire under load.
A bright blue, consistent spark indicates a healthy ignition system. A weak, orange spark or no spark at all points to a failing ignition coil, bad plug wires, or a faulty control circuit. Spark testers are essential tools for diagnosing crank-no-start conditions and ignition misfires.