Set the weed eater flat on a workbench.
Remove the spark plug wire from the spark plug located at the front of the engine. Remove the spark plug with the spark plug wrench and install a new spark plug (the new plug is pre-gapped), to ensure proper testing.
Insert the spark plug wire lead into the spark tester. Attach the alligator clip to the metal end at the tip of the spark plug. Turn the ignition switch to the ON position.
Pull the starter cord rapidly and watch the window in the spark tester. If the spark is strong and blue, the coil is operating properly. If the spark is weak or non-existent, the coil needs replacing.