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You will need to look at and/ or test these three areas if your microwave won't heat but you hear a loud buzzing or humming noise while it is running:
- Power diode
- High voltage capacitor
- or the Magnetron
CAUTION Your microwave oven is capable of giving you a serious electrical shock, even when it is unplugged.
Click here to learn more about each one of these items and to learn how to test them and replace them if necessary.
If the microwave sounds fine and there is no heat, one of the internal door switches, the high voltage circuitry, or the circuit board may be the problem and should be tested and replaced as necessary.
If you need further help or clarification on this answer, please do not hesitate to reply to this post.
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Best of Luck, Brian
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