The first thing you should know about spark plugs is that they are only used in petrol engines. Diesel engines use a different component called a glow plug.

How Spark Plugs Work

Spark plugs

The purpose of a spark plug is to repeatedly create a strong electrical spark. To understand why your engine needs a spark, you need to know a little bit about how an engine works.
How Spark Plugs Work - 4 stroke combustion cycle
There are 4 main steps that take place inside your engine to make it run:
  • Step 1) A mixture of petrol and air is sucked into the combustion chamber by the piston moving down inside the cylinder.
  • Step 2) The petrol & air mixture is squeezed in the combustion chamber by the piston moving up inside the cylinder.
  • Step 3) The petrol & air mixture is ignited by the spark plug. this causes an explosion inside the combustion chamber. The force of the explosion pushes the piston back down inside the cylinder.
  • Step 4) The spent gasses that result from the explosion are then forced out of the combustion chamber by the piston.
While the engine is running this whole process is repeatedly taking place thousands of times a minute. Interestingly, the sound that your engine makes is actually the sound of thousands of those little explosions taking place inside your cylinders. The animation below of a standard 4 cylinder engine is slowed down so that you can see the process
The video below probably makes it easier to understand. It shows what actually happens inside a real working engine. It’s slowed down dramatically so you can see what’s going on. First you can see the petrol & air mixture rushing in to the combustion chamber, through the inlet valve. The piston then comes into shot, compressing the mixture, the spark plug fires and ignites the fuel mixture, the force of the explosion pushes the piston back down, the exhaust valve opens and the piston pushes the old gasses out.
The spark plug converts electrical energy into a spark. The electrical energy comes from the battery, which is then amplified by ignition coils, it’s then routed to the individual spark plugs via ignition leads. The spark plug itself creates the spark by forcing the electrical energy to jump the gap between the central electrode and the earth electrode

 How Spark Plugs Work - cutaway

When should you change your spark plugs?
Spark plugs wear out and need to be changed. The recommended change intervals vary from car to car but as a rule of thumb it should be every 15,000-20,000 kms. Worn plugs will result in reduced fuel economy and reduced engine power.

Which spark plugs do I need?
The easiest way to select the correct spark plugs for your car is by using our reg lookup system. Our system will show you multiple brands of plug, all of which will be suitable for your car, you just need to check the descriptions carefully in-case of  limitations and then choose your preferred brand or price point.

How do I fit new spark plugs?
Fitting new spark plugs is one of the easier DIY jobs you can do with very little equipment or experience. The key points to remember are:

  • Take extra care not to cross thread the new plugs when fitting them, they should screw in with very little resistance
  • Don’t over tighten the plugs – Check the correct torque settings & use a torque wrench where possible
  • Make sure you re-fit the HT/ignition leads in the same position (it may help to number them)
Watch our ‘How To’ video for a full demo:
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