Microwave sparking inside: the causes, and the one rule to follow

Sparking inside a microwave — a flash, a crackle, a small blue arc, a smell of burning — is not something to watch and wait on. The rule is simple: press stop or open the door the moment you see it, switch off at the socket, and do not run the oven again until you know why. Ninety seconds of arcing will not hurt you through the door, but it can burn a hole in the cavity wall or destroy the magnetron, which is the most expensive part in the oven.

The good news is that nearly all sparking has one of four causes, three of which are cheap to fix and one of which you fix yourself in ten seconds.

Cause 1: metal in the cavity

This is the one you can sort out yourself. Look at what was inside:

• Aluminium foil, even a small torn piece stuck to food
• A spoon or fork left in the bowl
• A plate or cup with a gold or silver rim — the metallic paint arcs
• Twist ties, staples in a paper bag, the foil lid of a curd cup
• The metal grill rack used in a solo or microwave-only mode

Remove the metal, wipe any scorch mark, and the oven is fine. If your model has a grill rack, it is only for grill or combination mode; the manual says so because in microwave mode it arcs against the walls.

Cause 2: the waveguide cover

On the right-hand side wall, sometimes the roof, is a small rectangular panel that looks like thin cardboard. It is mica. It covers the opening where microwaves enter the cavity and keeps steam and splatter out of the waveguide and magnetron behind it.

Over months of reheating curry and dal without a lid, grease soaks into the mica. One day a splatter on the greasy patch burns, the carbon conducts, and you get a repeating spark in the same spot every time, often with a brown or black mark. Once it has burnt it will keep arcing; wiping it does not undo the damage. The cover is a low-cost part and swapping it is a short job. This is the most common repeat-sparking cause we see, and the one most often blamed on the magnetron by worried owners.

Cause 3: chipped paint or rust on the cavity wall

The inside of the oven is painted or enamelled metal. A chip — from a heavy dish dropped on the floor, a rack scraped along the wall, or abrasive scrubbing — exposes bare metal, and the edge of that chip arcs. In Nellore's humid air, and especially in coastal homes around Muthukur and Krishnapatnam, a small chip becomes a rust spot in a few months and the sparking gets worse.

A small arcing point can be cleaned up and touched in by a technician. A cavity with rust through to the outside is a different conversation — see our repair-or-replace guide.

Cause 4: running empty or nearly empty

With nothing inside to absorb the energy, the microwaves bounce back into the waveguide and magnetron. You may see arcing at the waveguide cover; you will certainly shorten the magnetron's life. Do not run it empty "to warm it up", and do not let a child press start on an empty oven to see the light come on. Reheating a single chapati on high for five minutes is close to empty by the end.

What to do right now, in order

1. Stop it: open the door or press stop.
2. Switch off at the wall socket and pull the plug.
3. Look for metal. Remove it. If that was the cause, you are done.
4. Look at the waveguide cover for a brown or burnt patch, and at the cavity walls for chips or rust. If you see either, do not run the oven again; book a visit.
5. Do not remove the outer cover to investigate. The high-voltage capacitor behind it can hold a dangerous charge for a long time after unplugging.

What the fix costs

A Sparking Inside visit is ₹350 on our rate card. It covers finding the arcing point, cleaning it and replacing the waveguide cover if it is burnt. The cover itself is charged at MRP and shown to you before fitting. If the sparking was left running and the magnetron has been damaged, the technician will tell you that before doing anything further, and the Not Heating diagnosis (₹399) applies instead.

Preventing the next one

Cover food when reheating, or use a microwave-safe lid. Wipe the cavity, including the waveguide cover, gently with a damp cloth once a week; never scrub the cover or the walls with a metal scourer. Check plates and bowls for metallic rims before they go in. And treat the first spark as a message, not a nuisance.