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Unfortunately this is a rather large problem. It means that either the cavity thermostat mounted above the microwave cavity has overheated and continuity broke so you need to replace the thermostat, or the aluminum foil caused the magnetron to overheat and fail and now the magnetron as well as the safety thermostat on the side of the magnetron will need to be replaced to fix the issue. These are not things that you should repair yourself because microwaves can store up to 3000 volts even when not plugged in which can be enough to kill you if the microwave is not handled properly.
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