My 3-year-old Frigidaire over-the-range microwave (FFMV162) suddenly won't stop heating until the door opens or it is unplugged. I can run it normally, say for a minute, and the turntable spins and fan blows and food gets hot. At the end of the countdown, it beeps like normal, and the turntable and fan stop, but a hum remains and the food keeps cooking. When I open the door the humming and cooking stop, but when I close the door it starts up again.

I've searched the forum here and saw other posts reporting this kind of problem. Usually someone recommends checking the door / interlock switches, and I think one person even responded back that this was the right fix for them. I don't understand why this is the case, though. The door is working normally best I can tell--it stops everything when I open it. Wouldn't the problem lie elsewhere, with some switch in between the control board and the magnetron, since it seems like the switch(es) between the control board and turntable and fan are working properly?