Spark plugs have an important task to perform in your engine. They produce an electrical spark that ignites the mixture of fuel and air in you engine's combustion chambers, causing the pistons to move up and down. This movement of pistons is what ultimately gets your car's wheels moving, so this little spark is a critical part of the whole process; it initiates the combustion in your internal combustion engine.
But like many parts of your engine, spark plugs won't last forever. Every time they fire, the spark removes some of the metal on the plugs, increasing the voltage necessary to fire the next time.
Besides age, many other things can cause your spark plugs to wear out. They're subjected to a number of forces inside the engine, including heat, gasoline and oil that can reduce their effectiveness. High performance engines and engines that rev to high RPMs also tend to wear out quicker.
For all of these reasons, it's important to replace your spark plugs from time to time. Fortunately, high-quality spark plugs should last tens of thousands of miles, so replacing them is not something you'll have to do often. Even if you don't put many miles on your vehicle, it's something that should be done at least every few years.
So how long do they typically last? Well, standard copper plugs are said to last anywhere from 10,000 to 20,000 miles (16,093 to 32,187 kilometers), while more expensive iridium or platinum spark plugs can last 60,000 miles (96,561 kilometers) or more [source: Spark plugs UK]. Some high-end iridium plugs are advertised to go more than 120,000 miles (193,121 kilometers), but there's no actual guarantee they'll be able to achieve this [source: AA1Car].
If you're not keeping track of your spark plugs' mileage, there are several symptoms to look for that can tell you it's time to have them replaced. If the engine has trouble starting, is rough at idle, stalls, or experiences a drop in fuel economy it may be time to have the plugs looked at by a mechanic. It's often easy to tell just from looking at the plugs whether they're worn out or not [source: Caradonna].
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