Sparking in microwave ovens is usually caused by exposed metal. Even a small piece of metal such as a staple from a food bag or steel wool left after cleaning can cause sparks. In addition, parts of the microwave itself can cause sparking if the paint inside chips or peels.
Should your microwave spark, turn it off, unplug it and check the interior carefully. If there’s only food inside the microwave when it sparks and you’ve cleaned it recently, check for any metal you may have put in along with the food. If you’re reheating food bagged at a restaurant, check to make sure there’s no staple in the bag.
When the paint inside a microwave oven chips or peels, exposed metal can cause the microwave to spark. The most likely parts to spark are the waveguide cover, turntable lugs and rack supports. You may be able to repair these parts with microwave oven paint. If the metal walls are sparking, you may be able to repaint them as well.
Microwave ovens also spark when their magnetron tubes – the component that generates heat – go bad. It is possible to replace the magnetron tube, but doing so requires close contact with the high-voltage components of the oven.