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So unfortunately this is quite a major issue. It means that your magnetron is arcing out and the magnetron at the very least would need to be replaced, plus if the wave guide is damaged through the frame of the oven, it may need to be re-painted so it doesn't arc out in the future. As for how long the unit should last, definitely longer than 2 1/2 years. But that being said there are very few moving parts on a microwave so just like a computer, I've seen them fail in the first month or I've seen them last 10 years with no problems---it's very sporadic and really not time dependent when they fail.
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