I'm not sure what you have tried, but maybe one of these tips will help you:

When foods are burned in the microwave, the plastic walls might absorb part of the burned odor. If you have burned food in the microwave, clean out the box with a solution made of one quart water, two tablespoons detergent, and 1/4 cup chlorine bleach. Be sure to wash the rubber gaskets and other items used in the microwave. If the odor persists, wash out again using a baking soda solution. Rinse with clean water and dry.

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Placing 1/2 cup lemon juice and heating in the microwave (then let set for a few hours) will remove the smell.

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Place three tea bags in 10 ounces of water and heat until boiled.

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Fill a large microwave-safe bowl with one cup of water and sprinkle your favorite spice like cinnamon or add several drops of vanilla or lemon extract. Bring to a boil. You must let it sit inside for 10 to 15 minutes to cool down. Simply wipe the walls down with paper towels. Leave the door open to further air it out. -Heloise Hints

To neutralize burnt food odors in the house, such as burnt toast, boil one quart water with two cups of vinegar in a heavy duty sauce pan. Simmer for 15-30 minutes. The smell disappears and the burnt odor is gone fast without chemicals. You can also put vinegar on pieces on white bread around the house to get rid of smells. source

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Set a saucer of ammonia in the microwave overnight, you can also set one out on your table or counter overnight, in the morning the house and microwave will smell much better (the saucer will still have liquid in it, but will not smell like ammonia).

:::I hope this helps:::