Spark Plug Test

SPARK PLUG TEST (WHEN ENGINE CAN BE CRANKED)
Connect the spark plug cable. Ground the outer electrode (main body), and crank the engine. In the atmosphere, only short sparks are produced because of the small discharge gap. If the spark plug is good, however, sparks will occur in the discharge gap (between the electrodes). In a defective spark plug, no sparks will occur because of a leak through the insulation.