Not a DIY fix
Microwave runs but doesn't heat
Lethal stored charge. This one is never a DIY fix.
The magnetron capacitor can hold a lethal charge for hours after unplugging, and the waveguide may contain beryllium ceramic. Do not remove the cover. Call a certified appliance repairer — or the manufacturer if it's under warranty.
Safer fixes to try instead
Anything on the green list needs no isolation at all.