Microwave emits sparks or arcing inside the cavity during cooking.

Safety first: Stop the microwave immediately and unplug it if you see sparks.

Step-by-step checks:

• 1. Remove metal objects: Ensure no metal utensils, foil, or twist ties are inside.
• 2. Inspect waveguide cover: Locate the mica or plastic waveguide cover on the side wall. If it is burnt, cracked, or has holes, it must be replaced.
• 3. Clean cavity: Thoroughly clean the interior with a non-abrasive cleaner to remove grease and carbonized food, especially near the waveguide cover.
• 4. Check enamel: Look for chipped enamel exposing bare metal. Small chips can be touched up with microwave-safe cavity paint; large damage may require cavity repair or replacement.

Do not operate the microwave with a damaged or missing waveguide cover; replace it with a Grundig-compatible part.