How do you test ignition coil on string trimmer?

To test the ignition coil on a string trimmer, first ensure the trimmer is turned off and disconnected from any power source. Remove the spark plug and connect it to the ignition coil wire, grounding the plug against the engine. Pull the starter cord to see if a spark is produced; a strong blue spark indicates a functional coil, while no spark suggests a faulty ignition coil. Additionally, you can use a multimeter to check the coil's resistance, comparing the readings to the manufacturer's specifications.