Stop the microwave, unplug it, and do not run it again until you know why it sparked. That is not a precaution written to cover us; a microwave that arcs can damage the inside of its own cavity in seconds, and damage to the cavity is what turns a small problem into a scrapped appliance.
Once it is unplugged, there are four causes and they split cleanly into two you can deal with and two you cannot.
And one thing that is never yours, at any point, for any reason: do not open the outer cabinet of a microwave. A microwave contains a high-voltage capacitor that can hold a dangerous charge after the appliance is unplugged. It is not like other kitchen appliances and there is no equivalent of "just have a look inside." Nothing on this page involves removing a cover.
The four causes
1. Metal in the cavity
The commonest by far and the least serious.
Obvious versions: cutlery, foil, a twist tie, a metal-rimmed plate or mug, a takeaway container with a foil lid, a chinese food container with a metal handle.
Less obvious versions: metallic decoration or gold banding on crockery, some recycled paper products, some plastic containers with metal-flecked finishes, and staples in a tea bag or a paper bag.
How to confirm it: with the microwave unplugged, take everything out and look at the plate, the bowl and the packaging. If you find metal, that was almost certainly it.
What to do: remove it, inspect the cavity for damage as described below, and if the interior is undamaged the appliance is usually fine.
2. Food debris or splatter
Burnt-on food, especially anything greasy or sugary, can char and then arc.
Where to look: the ceiling of the cavity, the walls, and around the waveguide cover, which is the small panel usually on one side wall.
What to do: with the appliance unplugged, clean it. The routine for that, including how to loosen baked-on splatter without scrubbing the interior finish, is in how to clean a microwave.
Do not scrape the interior. Removing the coating inside the cavity creates the next arcing site.
3. A damaged waveguide cover
This is the cause most owners have never heard of and it is the one that recurs.
The waveguide cover is a small panel, usually a thin light-colored board, covering the opening through which the microwave energy enters the cavity. Splatter builds on it, gets cooked repeatedly, and eventually it chars, blisters or burns through.
Symptom: sparking that happens repeatedly in the same spot, on the wall where that panel sits, with nothing metal in the oven.
What to do: this is a part, and whether it is owner-replaceable varies by model. Your manual is the document that tells you, and on many models it explicitly is not. Do not run the appliance with a damaged or missing cover.
4. The appliance itself
Arcing with no metal present, a clean cavity and an intact waveguide cover points at the appliance.
Symptoms that belong in this category: a burning smell, sparking accompanied by a loud buzz or hum that was not there before, sparking that continues after everything has been removed and cleaned, visible burn marks appearing on the cavity walls, or a unit that runs but does not heat.
What to do: stop using it, and get a qualified appliance technician or replace it. Not open it.
The inspection, with the appliance unplugged
Once it is unplugged and empty, look at four things.
The cavity walls and ceiling, for burn marks, blistered paint or bare metal showing through.
The waveguide cover, for charring, holes or a missing panel.
The turntable and its support, for anything metal that has come loose.
The door and its seal, for damage, and the door frame for anything bent or missing.
Any bare metal exposed inside the cavity is a stop. The interior coating is there for a reason and a bare patch will arc again.
The rules that are not negotiable
Do not open the outer cabinet. The high-voltage capacitor inside can hold a charge after unplugging. This is the one appliance in a domestic kitchen where "unplugged" is not sufficient protection for opening it, and it is why microwave repair is a specialist job rather than a general one.
Do not run it empty to test it. A microwave with nothing to absorb the energy is a bad situation for the appliance.
Do not run it with a damaged door or a damaged seal. The door is a containment component.
Do not "just try it once more to see if it does it again." Every arc is a chance to damage the cavity further.
Do not use a microwave with visible burn-through in the cavity wall.
Is it worth repairing
For a microwave the repair-or-replace question comes up faster than for almost any other appliance, because the labor to diagnose it can be a large share of what the unit is worth, and because the specialist handling requirement is real.
That said, the rule everybody quotes, that you replace when a repair costs more than half a new appliance, is widely attributed and could not be verified at a primary source. Our triage guide reports that trace honestly rather than repeating it as a published standard: is this a DIY fix or a service call.
One thing worth checking before you decide: whether the microwave is a countertop unit or built in. A built-in or over-range unit is a different proposition, because replacement involves the cabinet opening and sometimes the ventilation, and an over-range microwave is also doing the range hood's job. Whether ducted or recirculating matters there, and that is covered in ducted vs ductless range hood.
FAQ
My microwave sparked once. Can I keep using it? Not until you know why. Unplug it, empty it, and check for metal, for burnt-on food, and for a damaged waveguide cover. If all three are clear, treat it as an appliance fault and stop using it.
What is the waveguide cover? A small panel, usually on a side wall of the cavity, covering the opening where the microwave energy enters. Splatter builds on it and it can char or burn through, which produces repeated sparking in the same place with nothing metal present.
Can I replace the waveguide cover myself? It varies by model and your manual is the document that says. On many models it is not an owner part. Do not run the appliance with the cover damaged or missing.
Can I open the microwave to clean inside the casing? No. A microwave contains a high-voltage capacitor that can hold a dangerous charge after the appliance is unplugged. Nothing on this page involves removing the outer cabinet, and microwave repair is a specialist job for that reason.
Is a spark from metal foil serious? Usually it is the least serious of the four causes. Remove the metal, inspect the cavity for burn marks or bare metal, and if the interior is undamaged the appliance is generally fine.
There is a burning smell but no visible damage. What now? Stop using it and get a qualified technician, or replace it. A burning smell with no visible cause is in the appliance-fault category rather than the user-error one.
Sources: this page contains no voltage, cost, repair price or lifespan figure, and cites none. The waveguide cover replacement question is routed to the reader's own manual because it is model-specific. The capacitor warning is stated as an absolute prohibition rather than attributed to a figure, and the article contains no instruction that involves removing any cover. The unverifiable status of the widely quoted 50 percent repair-or-replace rule is carried from this site's own appliance triage article, which records the failed attribution trace.
