Your ignition system is pretty complex, and there are a number of important components that must work with one another to crank the engine and then keep it running. Your spark plugs are among the most important of these, and you have several of them (one per cylinder).

The job of your spark plugs

Spark plugs have a very simple yet essential job. They provide the spark necessary to ignite gasoline in the combustion chamber. Without working spark plugs, your engine will not run (there will be nothing to ignite the fuel and start the combustion process). Your car’s engine basically runs on a series of miniature, contained explosions. First, air and fuel are mixed in each cylinder. When the piston rises toward the top of the cylinder, the spark plug fires. Electricity arcs from the electrode to the tab, and the fuel and air mixture ignites. This creates pressure as the fuel is transformed into a gas (emissions), which pushes the piston back down and turns the engine. This cycle is repeated over and over again in each cylinder of your car.

The engine has one spark plug per cylinder. Each spark plug is connected to a spark plug wire (in conventional and electronic ignitions). The wire runs from the top of the plug to the distributor, which is attached to the coil. However, if you have a distributor-less ignition, you have no spark plug wires. Rather, coil packs are used and the whole system is controlled by the computer.

Over time, your spark plugs wear. This is completely normal. Eventually, they will need to be replaced. The primary form of wear is to the electrode – as the spark plug arcs, it loses tiny amounts of electrode material. Eventually, it shortens. When it becomes too short, it won’t arc properly and the engine will misfire.

This article originally appeared on as What Does a Spark Plug Do? and was authored by Valerie Johnston.

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