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if everything else work and it sounds like it usually does the magnetron has failed. this is what heats the food, this is not very expensive but is dangerous to replace if you don't know what your doing, the part is around $55.00 but unless your experienced I would get this replaced be a tech. even when microwaves are unplugged they store enough voltage to kill you,be safe. please call for a service tech to replace this, thanks Bryan