The Short Answer
Stop the microwave and unplug it. Look for metal — foil, twist-ties, a metal-trimmed dish, a forgotten fork — remove it, wipe the cavity, and test once. If it still arcs with nothing metal inside, the waveguide cover has almost certainly burnt through: a cheap part, but one behind the cavity wall that a technician should replace. Never keep running a microwave that sparks.
What to Do Right Now
Sparking is the one microwave symptom worth treating as an emergency, because arcing can scorch the interior or start a fire. The immediate steps: press stop, open the door, and unplug the unit. Give the cavity a moment, then inspect. Most first-time sparks have an innocent explanation sitting right there — a scrap of foil, a bowl with a metallic glaze, a bag tie — and removing it ends the problem. What you must not do is shrug and run it again to “see if it does it once more.”
What Causes the Sparking
| Cause | DIY? | Detail |
|---|---|---|
| Burnt or missing waveguide cover | No — technician part | The small mica panel on the cavity wall; the #1 cause of arcing when it chars |
| Metal in the cavity | Yes — remove it | Foil, twist-ties, metal-trimmed dishes, or a fork — take it out and retest |
| Damaged rack support or paint | Sometimes | Chipped cavity paint or a bent rack lets metal arc; touch-up or part needed |
| Dirty, food-crusted cavity | Yes — clean it | Heavy grease or char can ignite and spark; a proper clean often ends it |
| Failing magnetron / stirrer | No — technician | Less common, but internal faults can arc; needs professional diagnosis |
“Ninety percent of sparking calls are a two-dollar mica cover — the other ten percent are the reason we say don't gamble on it.”
Field note — The Technician Team
The Waveguide Cover Story
If the cavity is empty of metal and it still arcs, look at the small rectangular panel on the inside wall — often beige or grey, made of mica. That is the waveguide cover, and it shields the opening where microwaves enter the cavity. Years of grease and food splatter bake onto it, carbonize, and then arc like a tiny lightning storm. Replacing it is inexpensive, but it sits at the mouth of the waveguide, so a technician replaces it and checks that no arcing has damaged the cavity or the components behind it.
When It Needs a Technician
Book the visit if the sparking repeats with an empty, clean cavity; if the waveguide cover is visibly charred; if you see scorch marks on the cavity wall; or if the arcing came with a burning smell. Our microwave repair team in Sherwood Park replaces waveguide covers, inspects for deeper damage, and confirms the unit is safe before it goes back into service. If the same microwave also struggles to heat, pair this with the no-heat bulletin.
Sparking Questions, Answered
Is a sparking microwave dangerous?
Yes — treat it as a fire risk. Sparking (arcing) means microwave energy is jumping to something it should not, and it can scorch the cavity or start a fire. Stop the microwave, unplug it, and do not run it again until the cause is found and fixed. The good news is that the most common cause is a cheap, quick repair.
Why is my microwave sparking with nothing metal inside?
The usual culprit is the waveguide cover — a small mica or plastic panel on the interior wall that shields the opening where the microwaves enter. Grease and food splatter build up on it, char, and then arc. A burnt waveguide cover is inexpensive to replace, but it is behind the cavity wall and best done by a technician who can inspect for further damage.
Can I keep using my microwave after it sparked once?
Not until you know why. If you spot metal or a metal-trimmed dish, remove it, wipe the cavity, and test once — a single arc from foil is harmless once the foil is gone. But repeated sparking, or sparking with nothing metal inside, points to the waveguide cover or an internal fault, and running it risks a fire. When in doubt, book a technician.