Most microwaves today have a flame-oven thermostat-safety built into them. This oven thermostat will open up ( no continuity ) when something has been overheated or set on fire in the oven. An example of where the flame-oven thermostat is in an over the range micro, usually the thermostat for the oven is on the top area....

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The flame-oven sensor-thermostat is suppose to be closed ( full continuity ) all the time and will open up ( no continuity ) when needed, like when something like a small fire in the oven occurs.

A wiring diagram showing where the flame/oven thermostat may be.

A sample picture of a couple different flame/sensor thermostats