A spark plug is a key component of a petrol engine. As the name suggests, it provides the ignition between the compression and combustion stages of the four-stroke cycle used by modern car engines.
Most engines use one spark plug per cylinder, so you'll find two in Alfa Romeo and Fiat Twinair engines. Some, such as Mercedes M112 and M113 engines, have two spark plugs per cylinder, which makes replacement somewhat pricey because the M113 is a V8.
Find out how a car's ignition system works and what it includes here.
Watch this video to see how to change your plugs
What is a spark plug made from?
Modern spark plugs consist of ceramic and iridium or platinum. There's at least one - and often up to three - electrode(s) at one end and a terminal at the other. The electrode end sits at the top of the engine's cylinder, where it creates the spark, and the other is where the HT lead connects.
Learn more about what spark plugs are made from and what they do here.
Are bad spark plugs causing my engine to misfire?
Quite possibly, which is why it's worth checking them one by one and inspecting them for wear. In fact your spark plugs' tip and insulator can tell you a lot about your engine's health:
If they are clean and white, with no deposits, this is indicative of a weak mixture or too hot a plug (a hot plug transfers heat away from the electrode slowly, a cold plug transfers heat away quickly).
If the tip and insulator are covered with hard black deposits, then this is indicative that the mixture is too rich. If the plug is black and oily, then it is likely that the engine is fairly worn, as well as the mixture being too rich.
But if the insulator nose is covered with light tan to greyish brown deposits, then the mixture is correct and it is likely that the engine is in good condition.
If your spark plugs appear healthy, and the ceramic jacket isn't cracked, the HT leads or coil pack(s) may be to blame for the misfire instead. Find out more about the coil here.
What spark plugs do I need?
Every car is different and you can't just stick any old spark plug into the engine. Luckily, every Haynes Manual lists the spark plugs specs for your car, bike or truck, including the torque requirements for installation – including a handy tip to prevent cross-threading them.
See our Spark Plug Replacement feature, which forms part of our Home Mechanics Guides, here.