Why does my microwave spark inside?

Sparking in a microwave — arcing, properly — happens when the field concentrates at a sharp edge or a carbonised deposit until the air between it and something else breaks down. Three of the four causes are things you put in the cavity or let build up in it. The fourth is a fault, and the difference is easy to establish.

Safety

Step by step

1. Stop it and open the door.Do not let it run to see whether it settles. Arcing damages the magnetron and can set fire to food or packaging in seconds.
2. Look for metal, including the metal you did not think was metal.Foil, foil-lined packaging, a plate with a gold or silver rim, a twist tie, a fork left in the bowl, the metal clip in a takeaway container handle, a food bag closure, a takeaway box with a foil base. Any of these arc immediately and reliably.
3. Check the waveguide cover.This is a small rectangular panel, usually mica, on the right-hand cavity wall or the roof. It protects the opening the microwaves enter through. Food splatter builds on it, carbonises, and then arcs. Look for brown or black burnt patches or a hole.
4. Clean the cavity thoroughly, including the cover.Steam the cavity with a bowl of water, then wipe. The waveguide cover is wiped gently with a damp cloth and dried — never soaked and never scrubbed hard, because mica is soft and delaminates.
5. Replace the waveguide cover if it is burnt or holed.It is a cheap part, sold as a sheet of mica you cut to size, and it is one of the very few microwave parts an owner can legitimately change — it is inside the cavity, not inside the casing. Never run the microwave without one.
6. Check the turntable and any accessory.A crisper plate with a damaged metallic coating, a browning dish past its life, or a metal rack left in from a combination cycle all arc. Remove everything and test with a cup of water only.
7. Test with a mug of water in a clean, empty cavity.Half a mug of water, one minute on full. No metal, nothing else in there. If it arcs under those conditions, the fault is internal.
8. If it arcs clean and empty, take it out of service.That points at the magnetron, the waveguide itself or the cavity, and none of it is owner work. Given the cost of a microwave against the cost of that repair, replacement is usually the answer for a freestanding machine.

Tips

• Check the mica cover every time you clean the microwave. It is the part nobody looks at and the cause nobody guesses.
• Some dishes have a metallic lustre glaze that is not obviously metal. If a particular dish arcs and nothing else does, retire it from the microwave.
• Two flat metal items or two smooth metal spoons in a large bowl of liquid sometimes do not arc, which leads people to believe metal is fine. Sharp edges and thin foil always arc, and it is not worth learning the boundary experimentally.
• A microwave that has arced badly once may work perfectly afterwards. Clean it, replace the mica if needed, and watch it — but a repeat with a clean cavity is the signal to stop.

If it doesn't work