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I have a Toyota Etios Liva Viera Limited Edition car, 3 years old.

Last week, I took the car for the yearly service at the authorised service centre. The service person said that the spark plugs need to be replaced every three years or 100,000km (whichever is earlier). I had no objections to the replacement, because authorised service centre personnel surely knows more than me about my car.

But, why do I need to replace the plugs? Why won't they function properly after the specific period or distance? Aren't they only used in ignition of fuel? So, why change them periodically?

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Basically, over time they wear out. Here's an image of spark plugs old/new:

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Yours may not look exactly like either of these, but it gives you some indication. You have to realize electricity jumps the gap between the two parts (electrodes). This is kind of like a small welding arc being thrown between the two. Over time, this deteriorates the spark plug. Due to the larger gap, the ignition system in your car has to work harder to continue to jump the gap. This can lead to other issues, such as burning out ignition coils. It usually makes for worse fuel mileage as well. They'll usually continue to work as they grow old, but the thing is, they don't work as well. To keep your car running in tip top shape, periodic replacement is specified.

Feel lucky, with new spark plugs, the interval is as you suggest. Back in the day, you'd need to change them about every 20k miles.

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The spark plug surface tends to get pitted over a period of time, the gap between the points increases and there is carbon deposit on the surface. With the result that the spark generation deteriorates. While the gap can be set and points cleaned during service easily, over a longer period the pitted surface ensures that the gap is never uniform, hence an inconsistent spark. So periodic replacement is necessary.

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