Fortunately, it's possible to permanently remove the smell with just some basic household supplies you probably already have on hand.
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Fill a spray bottle halfway with vinegar. Fill the rest of the bottle with warm water, leaving a little room at the top. Replace the spray nozzle top securely and shake to mix.
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Open the freezer and remove everything. Dump the ice cubes in the sink or disposal, and discard old food items as you come across them. Temporarily place all sealed foods that you wish to keep in the refrigerator. Place any ice packs or chilled nonfood items in the sink for washing later. Remove any drawers or shelves and place them in the sink.
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Spray the vinegar solution liberally throughout the freezer's interior, including the top, sides, bottom and the inside of the door. Spray some of the solution directly onto a damp sponge. Wipe down all surfaces with the sponge. Continue cleaning until the inside of the freezer appears clean.
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Wash the ice cub trays, ice packs, shelves and drawers in a sink as you would if doing dishes. Sniff the ice cube tray afterward. If it smells of onions or other freezer smells, pour a small amount of vinegar into each section of the tray and allow it to sit for a while. Wash the tray again. Dry off the items that belong in the freezer.
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Return the washed freezer parts to their proper positions. Place the frozen food items back in the freezer, as well as ice packs and other frozen nonfood items. Refill the ice cube trays and return them to the freezer.