Using a spark test tool to diagnose misfiring cylinders, what indicates a misfiring cylinder?

• A. A cylinder that does not receive fuel
• B. A cylinder with worn piston rings
• C. A cylinder that does not fire with a strong, consistent spark
• D. A cylinder that does not fire with a strong, consistent spark or shows a weak spark.

Answer: (D)

A spark test tool checks ignition quality in each cylinder. For proper combustion, a cylinder must produce a strong, consistent spark to ignite the air-fuel mix. If a cylinder does not fire even when a strong spark is present, or if the spark is weak, that cylinder won’t ignite reliably and is effectively misfiring. The spark test doesn’t directly diagnose fuel delivery or mechanical wear—that’s why a choice mentioning only fuel delivery or piston wear isn’t the right indicator by itself. The best answer captures both no spark and weak spark as signs of misfire, which is why the option describing either condition is correct.