I had the same problem with my Whirlpool Microwave. It has done this off and on for several years. Usually unplugging it for a little while resolved the problem but not this time. I found that the R2 relay on the control board was stuck on. I removed it and soldered in wires to a 12vdc /30A automotive relay and the problem was solved. The relay I removed had a two-pin metal bridge that appeared to be just for mounting support but since it looked like it soldered into runs on the board, I added the yellow jumper wire. I also removed the diode from the board because the relay assembly I installed already had a diode.
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