Diagnosis
The smell is coming from the heating element chamber above the basket. Grease splatters up there and carbonizes.
How to Fix It
Unplug and cool. Remove the basket. Look up at the metal heating element shield. Use a soft-bristle brush (toothbrush works) dipped in a mix of warm water and baking soda. Gently scrub the shield and the surrounding cavity. Wipe with a damp cloth. Run the unit empty at 400°F for 10 minutes to burn off residue.
If That Didn't Work
For persistent smells, place a bowl of white vinegar or lemon water in the basket. Run a 15-minute cycle at 300°F. The steam will help loosen baked-on grease.
Heads Up
Never spray cleaner directly into the unit or submerge the main body. Do not use metal scouring pads on the non-stick coating inside the cavity.