Spark plugs operate in an extreme environment of high temperatures and high pressures. Over time they degrade and become less efficient.
A powerful electric spark is needed to ignite the mixture of fuel and air in your engine. If a spark plug is used too long it is less able to ignite this mixture, leading to an inefficient engine and lack of performance. This can cause potential engine damage and means you use more fuel.
We remove the spark plugs from the engine, inspect them (they can tell us a lot about how your engine's performing) and replace them with new ones.
We replace them at every major service.