This isn't totally true. As GPs get old the resistance starts to increase and you get a less than optimal heat from the plug. They do burn out like light bulbs but they don't necessarily burn at the optimum for their whole life. If you bench test and old GP you'll often find that it burns bright red to bright orange. A new one will be extremely bright orange to almost white. New ones will often leave sunspots in your eyes, whereas the older ones won't. Don't touch them though, they're HOT, obviously, something like 1600F.

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