Microwave turntable keeps turning when door opened

When I open the door of my ten-year-old Bosch microwave (HMT84M651B), the turntable continues to turn (but slowly). Everything I’ve read suggests it’s likely to be a problem with one of the three microswitches. I have tested them with a multimeter, both in situ (to check that the opening and closing of the MW door is opening/closing the switches) and disconnected. The switches themselves seem to be fine. However, the middle switch doesn’t seem to be doing anything, which can’t be right.

 It has three wires connected to it..

- a black one connected to the Common terminal and which goes to the Common of the top microswitch.

- a yellow one connected to the Normally Closed terminal (ie so it makes contact with the Common when the MW door is open) and which goes to a relay (302P-1AH-C-12VDC) on the control board.

- a red one connected to the Normally Open terminal (ie so it makes contact with the Common when the MW door is shut) and which goes to the big transformer that sits on the base of the MW.

 When I test the switch in situ with the three wires still connected to it, there is no resistance at all between the NC and NO terminals, regardless of whether the MW door is open or closed. After disconnecting the wires from the microswitch, it becomes clear that the microswitch is working properly, but there is no resistance at all between the red and yellow wires – they appear to be connected to one another at their other ends. Which means the microswitch is achieving nothing.

I wonder if this means the relay or something else has failed on the control board? Or maybe this “anomaly” wouldn’t occur if the MW had power? (I’m doing these tests with it unplugged, for the avoidance of doubt).

 Any comments and/or suggestions would be very welcome. Thank you.