Quite often this is due to grease or food particles stuck to the inside of the oven or stuck on or behind the waveguide cover.
The waveguide cover is usually about the size of a playing card and held in place by a plastic rivet.
It can be fragile, and the metal edges behind it can be sharp, so be careful!
The cover and the metal behind it should be cleaned with a mild, non-ammonia cleaner.
If the cavity metal of the oven is chipped or charred, it will have to be sanded smmoth and clean.
Then you can apply some high-temperature appliance paint, available from places liek Wal-Mart or Lowe's.
If your waveguide cover is burned, it will need to be replaced.
For more help from Emerson http://www.emersonradio.com ; Get help using your appliance 1-800-898-9020 ; Get help locating a servicer or parts 1-800-695-0098.
If it's not a problem caused by outside forces, it may be covered under warranty.
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