What is a waveguide cover?

The waveguide is the channel carrying energy from the magnetron into the cooking cavity, and the mica cover is the thin panel protecting its opening. Grease soaks into the mica, chars, and then arcs — producing the sparks and crackling that owners find alarming. It is a genuine wear item costing a few dollars, replaceable from inside the cavity with scissors and a screwdriver, and one of the very few microwave repairs an owner should attempt. Never run the oven with it removed.