The temperature sensor in the high voltage supply circuit needs to be replaced. It's part number is XXXXX and the price is $20.91. You can buy it at www.partselect.com.
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You need to remove the oven cover that goes over the sides and the top. After it's off, look at the oven's right rear area, behind the controls area. There you will see the magnetron power supply components. The part is located on or near the high voltage transformer.