The ignition module on a Stihl engine contains a coil that fires a high voltage spark up the lead wire to the spark plug. After multiple seasons of starting and restarting this ignition module will gradually lose its capacity to discharge the spark. Due to the high voltage discharge of this coil, electrocution is a serious danger if this is done improperly or with the wrong tools. Don't disassemble the ignition module if the coil is not firing properly as it can still carry a charge long after its last discharge.
Move the master control lever into the "Stop" position. Unfasten the rubber boot of the lead wire from the ending of the spark plug. Fit the 3/8-inch nut to the socket wrench and unscrew the spark plug. Take the old plug out and install the new spark plug. Tighten the new plug down firmly.
Insert the rubber boot onto the input terminal of the ZAT 4 ignition system tester. Push the tester's output terminal down firmly onto the end of the spark plug. Set the engine onto the ground.
Place your hand away from the ignition module, wires, and spark plug. Pull on the starter rope three or four times and look for a spark-over in the tester's window. If there's a spark, the ignition module is in working order.
Attach the spark plug's boot to the input terminal of the ZAT 3 ignition system tester. Attach the ground terminal on the ZAT 3 tester, using the alligator clip, to the tip of the spark plug. Set the adjusting knob to about 2mm. Place hands in safe areas away from the ignition system and tester.
Crank on the starter rope and check for a spark. If there's no spark, check the short-circuit wires and lead wires for damage and corrosion. Replace damaged wires and repeat the ZAT 3 test. If there's still no spark, replace the ignition module.
Always exercise extreme caution when troubleshooting the ignition system. The high voltage present in the ignition system can cause serious injury and even death.