To replace a microwave thermal fuse, unscrew the exterior panels, locate the fuse, lift it out, insert the new fuse, replace the exterior panels, plug in the microwave, and test the fuse. The supplies requires are a screwdriver, an ohmmeter, a new thermal fuse and a small food item.
Unplug the microwave, and unscrew its exterior panels. Lift them away from the microwave, and set them aside. Locate the fuse by following the power cord, pop it out of the terminal, and inspect it. If it is a glass fuse, look for black residue and a melted filament. These are symptoms of a blown fuse. If the fuse is ceramic, test it with an ohmmeter, as blown ceramic fuses do not blacken. A reading of "infinite" confirms that the fuse has blown and must be replaced.
Insert the replacement fuse. If the fuse feels loose or does not sit flush, squeeze the terminal clips until they wrap snugly around it. Reattach the exterior panels, and plug in the microwave. Place a piece of food or a container of water in the oven, and run the microwave for several seconds. Turn off the oven, and verify that the test sample is hot. Do not test the microwave while it is empty.