DIAGNOSIS The evaporator drip pan, located under the fridge near the compressor, has stagnant water growing mold or bacteria.
FIX Unplug the unit and pull it out. Locate the black plastic pan under the compressor area. It may be clipped or screwed in. Remove it. Clean it with a mixture of warm water and baking soda or mild detergent. Rinse and dry completely before reinstalling.
IF THAT DIDN'T WORK The odor may also be from spilled food inside the fridge compartment. Remove all shelves and drawers. Clean interior surfaces with a solution of 1 tablespoon baking soda per quart of water. Do not use harsh chemicals.
HEADS UP Never use bleach or ammonia to clean the interior or drip pan. Fumes can corrode the sealed cooling system and the smell can permeate plastic, making it worse.