Microwave Sparking Inside? Here's What to Do

Seeing sparks inside your microwave is alarming --- and you should stop the microwave immediately when it happens. But before assuming the appliance is broken, know that sparking has several causes, most of which are straightforward to address. Here's how to identify and resolve the issue.

Stop the Microwave Immediately

The moment you see sparks, arcing, or a small flame inside the microwave, press Stop/Cancel and open the door. Do not continue running a microwave that is sparking --- the electrical arcing can damage the waveguide cover, the interior walls, and ultimately the magnetron. Let the unit cool for several minutes before inspecting.

1. Metal in the Microwave

This is the most common cause of sparking --- and it has nothing to do with a malfunction. Metal objects, foil, twist ties, metal-rimmed dishes, stainless steel travel mugs, and utensils all cause sparking when exposed to microwave energy. The metal creates an electrical arc (the spark) as current tries to pass through it.

Fix: remove the metal item, wipe out any carbon deposits on the interior walls with a damp cloth, and inspect the interior for any burns or damage. If the walls look intact, the microwave is fine to use.

2. Dirty or Burned Waveguide Cover

The waveguide cover is a flat panel (usually mica or plastic) on the interior side wall of the microwave. It protects the internal electronics from food splatter while allowing microwave energy to pass through. When food residue builds up on the cover and gets burned by microwave energy, it can cause sparking and arcing --- even with no metal present.

Inspect the waveguide cover for brown stains, burned spots, or holes. A lightly soiled cover can be cleaned with a damp cloth. A cover with burn marks or holes needs to be replaced --- they are inexpensive ($5--$15) and available for most models. Never run the microwave with a missing or heavily damaged waveguide cover.

3. Food Residue on Interior Walls

Splattered food --- especially high-fat or high-sugar foods --- can build up on the interior walls over time. When microwave energy hits concentrated residue, it can cause arcing or sparking. This looks like small flashes from the wall rather than from any specific object.

Fix: thoroughly clean the interior of the microwave, including the ceiling, side walls, door interior, and turntable. Use warm soapy water or a microwave-safe cleaning solution. A bowl of water with lemon juice run on high for 3--5 minutes loosens stubborn residue and deodorizes at the same time.

4. Damaged Interior Coating

The interior walls of a microwave are coated with a paint or enamel that reflects microwave energy. When this coating chips, scratches, or peels --- sometimes from aggressive cleaning, sometimes from age --- exposed bare metal can cause arcing. Look for chips or peeling paint on any interior wall surface.

Minor chips can be addressed with microwave-safe touch-up paint available from appliance parts suppliers. Large areas of damaged coating or damage on the ceiling of the cavity are more concerning and warrant a professional assessment.

5. Failed or Damaged Magnetron

In some cases, sparking originating from the back wall or the area around the magnetron assembly indicates magnetron failure or damage to the magnetron antenna. This type of sparking comes from inside the microwave's internal components, not from food or the waveguide cover.

If you've confirmed there's no metal, the waveguide cover is clean and intact, and the interior walls are undamaged --- yet sparking continues --- the magnetron or its mounting may be the problem. Stop using the microwave and have it professionally inspected.