Microwave sparks or arcs inside

Q: Microwave sparks or arcs inside

In Canada, when a microwave sparks or arcs inside, stop it immediately — running it while it arcs can damage the oven or start a fire. The most common cause is metal in the oven: foil, a fork, a metal-trimmed dish, or a twist tie. Next most common is a dirty interior, with food or grease on the walls or roof arcing, or a greasy or burnt waveguide cover (the small panel inside). Chipped or burnt interior paint exposing bare metal can also arc. If it arcs with nothing metal inside, or you see smoke or flames or smell burning, unplug it and don't use it — the high-voltage parts inside hold a lethal charge and are a licensed appliance technician's job, never a homeowner repair. Because many microwaves are inexpensive, replacement is often the sensible answer.

The first move is simple: stop the microwave the moment you see arcing — don't keep running it. Most of the time the cause is something in the oven you can see and remove, not a broken machine.

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In Canada, check for metal first. Stop the microwave, then look inside for foil, a fork or spoon, a dish with a metal rim or gold trim, a twist tie, or a takeout container with a metal handle — any of these will spark. Remove all metal and try again with just microwave-safe dishes. This is the most common cause by far and costs nothing to rule out.

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handymanHow to fix it

Stop the microwave, remove every piece of metal from the oven, and reheat using only microwave-safe dishes — no foil, metal-rimmed or gold-trimmed dishes, metal utensils, or twist ties.

Stop the microwave the moment you see a spark — don't let it keep running.
Open the door and remove anything metal: foil, forks or spoons, a dish with a metal rim or gold/silver trim, a twist tie, or a takeout container with a metal handle.
Transfer the food to a plain microwave-safe glass, ceramic, or plastic dish and reheat. If the arcing is gone, metal was the cause.

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Removing metal from a microwave is a normal user task, permitted everywhere in Canada.

handymanHow to fix it

Unplug the microwave, then wipe the entire interior — walls, floor, roof, and door — clean of baked-on food and grease with a damp cloth and mild dish soap, dry it, and test.

Unplug the microwave at the wall.
Wipe the interior walls, floor, roof, and inside of the door with a cloth dampened with warm water and mild dish soap; for stuck-on splatter, microwave a bowl of water first (plugged back in) to steam it loose, then unplug and wipe.
Dry the interior fully, plug back in, and run a short test with a cup of water. No arcing means the char was the cause.

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Cleaning the interior cavity, unplugged, is a normal user task, permitted everywhere in Canada.

visibilityHow to check the waveguide cover

warning Unplug it before you reach insideEven to look at or clean the cavity, unplug the microwave at the wall first — and never remove the outer cabinet. The high-voltage capacitor behind that cabinet can hold a lethal charge for a long time even after unplugging, which is why anything past the cavity is a licensed technician's job. Cleaning and swapping the waveguide cover from inside the cavity, with the unit unplugged, is safe for a homeowner.

Unplug the microwave at the wall before reaching inside — never work in the cavity while it's plugged in.
Find the waveguide cover: a small mica or thin cardboard-looking panel, usually on the right-hand interior wall or on the roof, held by a tab or a single screw.
Shine a light on it and look for grease buildup, dark scorch marks, burnt spots, cracks, or holes. Light grease means clean it; charring, holes, or warping mean replace it with the matching part.

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handymanHow to fix it

With the microwave unplugged, gently clean a lightly greasy waveguide cover in place; if it's charred, cracked, or holed, remove it and fit a matching replacement cover cut or sized to the original.

Unplug the microwave at the wall.
For light grease only, wipe the cover gently with a damp cloth and let it dry fully before use — don't scrub a mica cover hard, it's fragile.
If it's scorched, cracked, or holed, release the tab or single screw and remove it. Fit a replacement mica cover sized to match — many are a universal sheet you cut to the old one's shape and hole position.
Reinstall so the shiny side faces the cavity, plug back in, and test with a cup of water.

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Microwave waveguide cover (mica sheet)Match or cut to the original's size and mounting hole; universal mica sheets are sold at appliance-parts retailers

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Cleaning or replacing the waveguide cover from inside the cavity, unplugged, is owner-permitted everywhere in Canada; no licence or permit applies.

handymanHow to fix it

warning Only microwave-safe paint, and never open the cabinetTouch-up here means the interior wall only, with the unit unplugged, using paint sold specifically for microwave cavities — ordinary paint can char or catch fire. Don't sand into or remove the outer cabinet to reach anything. If the metal is rusting through or the damage is extensive, the safe answer is a new microwave, not a repaint.

For minor chips only, touch up bare metal with paint specifically sold as microwave-safe (microwave cavity paint) and cure per its label. If metal is rusting or damage is widespread, replace the microwave rather than repaint.

Unplug the microwave and judge the damage: a small chip on an otherwise sound wall may be touch-up-able; rust, bubbling over a wide area, or damage near the waveguide opening means the oven is at end of life — replace it.
If touching up, use only paint labelled microwave-safe / microwave cavity paint. Ordinary spray paint or appliance enamel is not safe inside a microwave.
Lightly clean the spot, apply thin coats per the product label, and let it fully cure for the stated time before plugging in and testing.

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Microwave-safe cavity touch-up paintMust be labelled for microwave interior use; do not substitute ordinary appliance or spray paint

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Microwave-safe interior touch-up, unplugged and without opening the cabinet, is owner-permitted everywhere in Canada; no licence or permit applies.

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A microwave's high-voltage capacitor can store a lethal electric charge for a long time even after the unit is unplugged. Opening the outer cabinet, or touching internal parts like the magnetron, diode, or capacitor, can be fatal. This is why internal arcing is never a homeowner repair. Unplug the microwave, leave it closed, and call a licensed appliance technician — or, given how inexpensive many microwaves are, simply replace it.

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Opening the cabinet and servicing the high-voltage components is restricted to a qualified/licensed appliance technician everywhere in Canada on safety grounds — the capacitor holds a lethal charge.

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