When you turn the key in your 2007 Subaru Forester’s ignition switch, it begins a long and complicated process that eventually results in your engine starting up. Along the process, there are a massive number of components required to work properly, and some components even rely on others to work, so many problems can be caused by even the simplest problems. One of the parts required in this process is the spark plug.
The process we spoke of earlier results in the fuel from your fuel tank being pumped into a cylinder, provided with an ignition source and then burnt until it becomes the power to turn the wheels. This ignition source is a spark and, as you might have guessed, this spark comes from one spark plug in each cylinder. When it is provided with power from the distributor or ignition coil, it will instantly spark and cause the cylinder to 'fire' at exactly the right time.
The spark plug is a small component because it needs to be able to fit inside a relatively small cylinder. It is long, like a pencil, with a 'screw type' terminal at one end where it can be screwed in and fitted securely so it does not go anywhere under the serious vibrations of the engine. The filament at the other end is a simple component that turns electricity into a spark of flame, and this flame will ignite the fuel when it is pumped into the cylinder at exactly the same time.
While it is a relatively simple component without the complexity of some others, there are still problems that can occur. Its biggest problem is time, and after a period, it will become damaged simply by wear and tear. The damage mostly occurs to the sparking filament, and will prevent it from being able to spark so the fuel cannot be burnt and power cannot be supplied. Other issues can happen with the spark plug wires, or the internals inside the spark plug, which can all be affected by the heat and vibrations.
When one or more of these problems does occur, there are some symptoms to show you that you need to begin looking for more spark plugs. Symptoms include things like a rough idle, and it can begin to get rougher and rougher when it starts with one broken spark plug and ends with more than one. If the timing of the plug is out by the tiniest fraction, you can see something called 'pre-ignition,' which means the engine continues running after you turn the key to 'off,' or the engine simply will not start because no fuel can be burnt.