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It's the other way around. Charmander is likely to be dead soon if his flame is about to go out on itself. As you man know already, the strength of his tail flame represents his health level.

I've read about this subject many times before on the net and I think there was a bit about extinguishing Charmander's tail fire while he's healthy. It would simply light up again shortly afterwards. So his tail fire's kinda like a trick candle that doesn't go out (if you remember those from your childhood).