At spark plugs they are what make it possible to ignite the air/fuel mixture in the combustion chamber by means of an electrical spark. Don't wait for the first warning signs to change them. As a general rule, the car manual stipulates the maintenance period for the engine spark plugs depending on a certain mileage, a value that varies depending on the vehicle.
However, in most manuals there is also a recommendation to cut usage by half if the vehicle is subjected to intensive city use — after all, when the vehicle is stopped in traffic, the engine continues to run. In other words, if the manufacturer recommends changing the spark plugs every 30 000 km, they must be replaced every 15 000 km.
Why is it so important to anticipate candle wear?
In addition to loss of performance and potentially increased fuel consumption, worn spark plugs can compromise the catalyst and oxygen sensor, costly repairs to the wallet and avoidable. In case of doubt, it is recommended to inspect the spark plugs every year or every 10,000 km.
The ideal is to look for a mechanic or a specialist you trust, who will be able to tell you whether or not the spark plugs can be used for a while longer. If you want to change the spark plugs yourself, you can do it — it's a relatively easy operation, it all depends on your mechanical skills (the generations that used to ride “DT 50 LC” and “Zundapp” shouldn't have much trouble ).
The exchange must be done with the engine still cold and you must be careful not to damage the cylinder head threads.
And the Diesels?
Everything that has been said here is valid for gasoline engines, which depend on spark plugs for combustion. In the case of Diesel engines, the case changes. Although these also use candles, these are pre-heating.
The operating principle of the diesel engine is different — diesel combustion takes place by compression in the combustion chamber and not by spark. Hence, spark plug problems are more critical and recurrent in gasoline engines.