The average life of spark plugs depends on a number of factors: the mechanical condition of the engine, the type of fuel, driving conditions and the skill of the driver. Although a standard factory plug will last a considerable period of time, extended life can be achieved on some engines by using platinum-tipped plugs. Decide for yourself whether the extra cost is worth it.
When removing spark plugs, check their condition. The condition of the plugs is a good indicator of the condition of the engine. A slight deposit of light yellow or gray material (or rusty red on unleaded fuel) on spark plugs that have been in use for some time should be considered normal. Any other color, or abnormally large amounts of deposit, indicate that something is wrong with the engine.
The gap between the central and side (ground) electrodes increases slightly during normal operation.
When spark plugs are functioning properly, or more accurately, when the plugs are installed in an engine that is functioning properly, the plugs can be removed, cleaned, gapped, and reinstalled in the engine without any harm to the engine's operation. This is acceptable for some improvement in the plugs' performance or, in extreme cases (e.g., when there is nothing to replace them with), but new plugs are always preferable.
If the spark plugs are clogged (no normal spark), you need to find the cause (or source) of the contamination, eliminate it, and clean or replace the spark plug. Replacement is always recommended if possible.
The different spark plugs suitable for use in your engine are designed to operate in different temperature ranges. The amount of heat the plug can absorb is determined by the length of the lower insulator. The longer the insulator, the higher the temperature the plug will operate at; the shorter the insulator, the lower the temperature. Spark plugs that absorb little heat and run too cool will accumulate lead, oil and carbon deposits because they are not hot enough to shed (desorb) the deposits. This leads to fouling of the plugs and, subsequently, poor ignition performance. Spark plugs that absorb too much heat will not accumulate deposits, but the electrodes will wear too quickly and, in some cases, overheating can lead to premature ignition.
Pre-ignition occurs when the spark plug tips become so hot that they ignite the air/fuel mixture before the spark actually occurs. Pre-ignition usually causes a knocking sound at low speeds and under heavy loads. In the most severe cases, the heat can become great enough to cause the air/fuel mixture to burn throughout the entire combustion chamber, rather than just at the front of the spark plug. In this case, the explosion is strong enough to damage pistons, rings, and valves.
In most cases, the manufacturer's recommended temperature range is correct; it is selected to provide adequate performance over a wide range of operating conditions. However, if most of your driving involves long distances and high speeds, you may want to set your spark plugs one step colder than the recommended standard. If most of your driving is short and involves a variety of conditions (such as start-stop) where the engine may not always reach operating temperature, a hotter plug may help burn off the deposits that tend to accumulate under these conditions.