To my experience, this is one of the toughest problems to solve.
I have been called out on a variety of “bad odor” complaints, and rarely are they solvable. Modern fridges, with few exceptions, have plastic interiors that do absorb odors to some degree, rather than the porcelain interiors standard in days of yore. If you have an odor problem, it may help to keep an open jar of baking soda in there, or to wash out the interior of the fridge with a very mild bleach solution.
A lot of people use baking soda. In my opinion, baking soda is ok for small food odors, but it really isn’t made for bigger odor problems.
Dry, black, activated charcoal is much better for controlling normal odor issues. You can get it at pet stores; it is used to filter aquarium water.
There is a product called Ozium that does a really good job on tougher odors. It looks like an air freshener, but it is not. It actually destroys airborne bacteria. You can get the scented stuff, but I prefer the unscented spray. Make sure you use it as directed on the can…don’t breathe it directly. A one-second puff is all you need…you can repeat if necessary.
If your fridge has had a meltdown (thawed out with everything in it), try pulling off whatever panels you can and see if any meat blood or other smelly stuff has gotten under them. Styrofoam panels can absorb odors, too, but try washing with a mild soap solution.
One semi-solvable odor problem that I ran into was a fellow whose fire-engine red fridge was getting warm and it had an absolutely acrid smell within. It turned out that the defrost heater lead within the defrost compartment had shorted out and burned off a bunch of its heavy rubber insulation, then melted itself so the defrost system was not working. I fixed the defrost heater and used lots of Ozium, sold him some Ozium and told him how to use it, and told him about activated charcoal. But to be honest, I do not know if the burnt-rubber smell came back. It was pretty acrid.