Diagnosis
Grease and food particles get pulled into the fan area during cooking and get baked on by subsequent heat. It's a tough spot, but you've got to clean it to prevent smoke and odors.
How to Fix It
The key is using steam to loosen the grime. 1) Unplug the oven and let it cool. 2) Fill a oven-safe bowl with water and a tablespoon of white vinegar or lemon juice. 3) Place it on the bottom rack. 4) Manually set the oven to bake at 250°F for 20 minutes. The steam will soften the baked-on grease. Let it cool slightly, then unplug it again. Wearing a rubber glove, use a non-abrasive scrub pad (like a blue Scotch-Brite) dipped in warm, soapy water to gently wipe the back wall and fan blades. Avoid harsh scrapers.
If That Didn't Work
For really stubborn spots, make a paste of baking soda and a little water. Apply it to the greasy areas and let it sit for 15-20 minutes. The baking soda will help lift the grease. Wipe clean with a damp cloth.
Heads Up
Never spray oven cleaner directly into the oven, especially near the fan housing. The caustic chemicals can damage the heating elements, the fan motor, and the non-stick interior coatings. Stick to the steam and baking soda methods.