Diagnosis
Burned-on food deposits on the stainless steel interior. The stain is on the waveguide cover, a critical component on the ceiling of the cavity.
How to Fix It
Unplug the unit. Place a microwave-safe bowl with 1 cup of water and slices of lemon inside. Heat on High for 3-5 minutes until the cavity is steamy. Let stand for 5 minutes. The steam will loosen deposits. Wipe the interior, including the waveguide cover, with a soft cloth dampened with warm water and mild detergent. Do not scrub the waveguide cover aggressively.
If That Didn't Work
For persistent stains, make a paste of baking soda and water. Apply it gently to the stained area on the side or back walls, avoiding the waveguide cover. Let it sit for 10 minutes, then wipe clean. Rinse with a damp cloth.
Heads Up
Never use abrasive cleaners, steel wool, or harsh chemicals on the stainless interior or the waveguide cover. Do not allow cleaning solution to enter the vent openings on the top or sides. The waveguide cover must not be punctured.