What Is a Spark Plug?

The zap of electricity that initiates combustion in your engine comes from this small but mighty part.

A gloved hand holds a spark plug above an engine bayAustin Lott | Capital One


In gasoline engines, spark plugs create a spark of electrical power to ignite a fuel/air mixture within the engine's cylinders. This sets in motion a series of mechanical movements that keep your car moving.

Spark plugs are a small but vital part of your car's engine and ignition system, so periodic maintenance and replacement is necessary to keep your vehicle running efficiently.

Spark Plugs Create Miniature Explosions to Power an Engine's Cycle

Spark plugs create the carefully timed explosions that power the engine cycle. An arc of electricity travels across the gap between two electrical leads on the plug, sparking an explosion at the peak of compression inside your engine's cylinders. The explosion blasts the piston back down to turn a crankshaft, supplying power. Each cylinder has at least one spark plug.

To withstand creating millions of 20,000-volt to 100,000-volt explosions, spark plugs need to be made of long-lasting materials. Basic spark plugs are made up of a copper electrode coated with nickel alloy to protect it, surrounded by a shell of steel wire. That's encased in an aluminum-oxide ceramic insulator, which helps prevent electrical discharge outside of the piston's combustion chamber.

Other more expensive metals, such as platinum and iridium, can be used for the electrode tips on longer-lasting spark plugs.

Some Spark Plugs Can Last More Than 100,000 Miles

Modern electronics and manufacturing standards mean that platinum-infused spark plugs are designed to last about 50,000 miles, and iridium spark plugs can have an operational life of more than 100,000 miles. Copper plugs last only 10,000 to 20,000 miles.

If you notice issues such as hard starting, a rough idle, or slower-than-normal acceleration, worn-out spark plugs could be the culprit. Higher-than-normal gas consumption can be a giveaway, and old spark plugs may trigger a check engine light.

Overheating, an incorrect fuel/air mixture, and an improper gap between the electrodes can shorten the life of a spark plug. This gap is checked with a special tool that sets the correct distance.

Spark Plugs Can Be Cleaned or Replaced

Replacement of spark plugs is an integral part of a tuneup. Having your spark plugs inspected — and making sure the electrical wiring connections in your engine are up to snuff — can help determine if they need to be replaced. Spark plugs can be cleaned with a wire brush or a spray-on cleaner to help get rid of some of that carbon fouling. This is also a good time to ensure the electrodes are properly gapped.

While spark plugs and a gapping tool are relatively inexpensive, accessing spark plugs can be difficult. In some cars, spark plugs are located deep in the engine bay, and special tools may be needed to reach them.

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Andy Stonehouse
Andy Stonehouse literally fell into the world of auto writing while working as a ski-town journalist, and has not looked back since. A childhood spent dealing with the eccentricities of a 1976 MG Midget has made any subsequent auto experience a more safe and reliable drive. He has been blessed with nearby mountain trails and snowy roads in Colorado to do TV-adventure-styled test drives on a weekly basis.