The spark plugs are car parts which can be bought from Teimo's Shop for 99 mk per spark plug box, each box contains four spark plugs. They are attached to the cylinder head with the spark plug wrench/socket.
Spark plugs will become worn over time when the engine is running, and need to be replaced regularly. The condition of a spark plug can be checked by unscrewing it from the cylinder head and looking at the tip, there are three different visual wear levels:
- 65% - 100%: The tip of the spark plug is shiny, and it doesn't need replacing.
- 15% - 65%: The tip of the spark plug is not shiny anymore, and it likely needs replacing.
- 0% - 15%: The tip of the spark plug is completely black, and it requires replacing.
When one or more of the spark plugs are worn/broken, the engine has a rough time idling — typically stalling almost immediately whenever the engine isn't being revved — and power is not constant due to misfiring.
Some other problems can cause the same symptoms as worn spark plugs, such as heavily damaged pistons.