Replacing a fuse
Locate your fuse. Most microwave fuses look like the one pictured to the left. They are usually an inch or two long. Each end is metallic and the center is either ceramic or glass.
To get to the fuse, remove the outer cabinet of the unit, which is usually held in place with a few screws. Once the outer cabinet is removed, follow the power cord into the unit. It will take you to the fuse holder, where the fuse sits. The fuse holder is located two or three inches into the unit, and may be attached to a power board.
Decide if your fuse is blown. A blown glass fuse will produce flash burn marks on nearby surfaces, or it will be completely black on the inside. The internal fuse wire (filament) will be melted and blackened. Ceramic fuses give no outward signs of having blown. To determine if a ceramic fuse is blown, test it with an ohmmeter. If the ohmmeter reads zero ohms, your fuse is fine. A reading of infinite ohms means your fuse is blown.
Replace the blown fuse. Pop it out of its holder and snap in a new one. You can squeeze the fuse clips together with your fingers in order to tighten their grip. Replace the unit's outer cabinet, plug it back in, and test to see if it is running properly. DO NOT run the microwave with nothing in it.
NOTE: Fuses do not usually blow on their own. A fuse may blow because of problems with the interlock switches or with high voltage circuitry. It is also possible, though rare, that a power surge will cause a fuse to blow.