The problem may lie with the aroma of, for example, spoiled or decomposed food/meat products. Such a bad smell will linger even if you clean the fridge with bleach. That scent will not leave because of the styrofoam used to insulate freezers being very porous and inaccessible to the common cleaners we can use. If you can remove that foam and clean it or replace it - the smell would leave. I know - I had a freezer which broke and the melting of the meat products left it smelling like a morgue. We opened it and found the above scenario - which alas - led me to a new and better fridge. If you can use something to penetrate the foam or lining - you could remove the scent - otherwise - you need to replace the unit.