This is a combination microwave-convection oven. At times it works perfectly and at others it will not turn on. I believe the problem is in either or both of the door switches. If I look up the parts, listed are two switch interlocks Part AP2024367 @$18.29 each, a switch monitor AP2024325 $17.69 and a switch lever Part AP2016540 $2.89. How can I tell which one(s) I need to replace. My guess is the switch monitor but is that the most likely since the problem is intermittent.