Spark Plug Test

Remove the spark plug and connect to the ignition coil.

Fig 1: Identifying Spark Plug
Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
Ground the spark plug outer electrode (body), and crank the engine.
Check that there is an electrical discharge between the electrodes at this time.

Fig 2: Identifying Defective Insulation Of Spark Plug
Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.