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?