Microwave ovens are becoming cheaper every day. Unfortunately, though, they don't seem to last as long as they used to. One of the common failures in a microwave is sparking and burning of the waveguide cover. This is a small plastic or mica cover on the side or top of the cavity. The trick to prevent this is to always keep it clean. Most people wipe over the inside, but this cover can get grease and food on the other side as well, and this eventually burns, damaging the cover. While it is not an expensive part, by the time you add labour it is often enough to write the appliance off. The solution is simple. Remove this cover once in a while and wipe both sides. As a technician, it saddens me to see so many things end up in landfill prematurely.