A spark plug screws into the cylinder head and fires a small lightning bolt across a gap. That spark ignites the fuel-air mixture. Without it, a petrol engine does not run.

The plug has a central electrode, a ground electrode, and a ceramic insulator. High voltage from the ignition coil jumps the gap, creating a plasma channel hot enough to start combustion. The ceramic insulator keeps the voltage from grounding out before it reaches the tip. Materials matter: copper cores conduct well, iridium and platinum tips last longer.

Reading a spark plug

• Light tan or gray insulator: healthy combustion.
• Black soot: rich mixture or weak spark.
• White or blistered: lean mixture or overheating.
• Oil fouling: worn valve guides or piston rings.
• Electrode wear: gap widens over time, misfires follow.

Spark plugs are cheap and easy to replace on most engines, though some modern designs bury them under intake manifolds. Manufacturers specify replacement intervals, often 30,000 to 100,000 miles depending on the metal. A misfiring plug wastes fuel and can damage the catalytic converter. It is one of the few parts a weekend mechanic can still diagnose by eye.