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If the unit shuts off after a few seconds, it means that the microwave doesn't detect the magnetron drawing power. This can be caused by a bad magnetron, a bad inverter board, or one of the 3 door switches failing. You can't test the inverter board or magnetron, but you can test the door switches. You'd need to take the microwave apart, remove the control panel, and test the 3 door switches when you open and close the door to make sure that the switches operate properly. If they ALL work properly, you'll need to order and replace either the magnetron or the inverter board. Since you can't test them it's a bit of a guess but the inverter board is usually the part that fails so that is the part I'd replace first. Below is a link to the part.
CLICK HERE for inverter control board
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