The short answer
On a built-in machine the cavity is harder to see into and the waveguide cover is the part that quietly takes the damage. Ranked first on this platform: A burnt or soiled waveguide cover. The checks below follow that ranking, and the first of them need no tools and no panel removed.
What you observe
- Repeated arcing from the same spot on one wall, with a scorched panel there — A fixed location plus a scorch mark is the cover.
- A bright flash and a crack, with a specific dish or wrapper in the appliance — An arc tied to one dish is that dish.
- The breaker goes the instant cooking starts, repeatably — A trip tied exactly to the start of cooking is the appliance, not the circuit.
- NEAR MISS — the breaker trips only when several appliances run together — That is a circuit loading question rather than an appliance fault, and it is answered by what else is on the circuit.
Causes, most likely first on this platform
- A burnt or soiled waveguide cover — The small panel on the cavity wall covers the opening the energy comes through. It ranks first on this platform because splashed food carbonises on the cover, carbon conducts, and once it does the cover arcs every time the appliance runs — harder and blacker each time, which is why a scorched cover is a replacement rather than a wipe-and-carry-on.
- Metal in the cavity — Foil, a metal-trimmed plate, a twist tie or a fork produces arcing immediately. It ranks second here because the tolerances on this platform are tight enough that a fault which would be inaudible on a mass-market machine is obvious on this one.
- The appliance tripping its protective device — A microwave draws heavily, and a fault on the high-voltage side usually announces itself by tripping a breaker at the moment cooking starts. Ranked third: worth checking once the causes above have come back clean.
- A damaged cord, plug or outlet — A cord crushed behind a cabinet, a plug that has been running warm or a tired outlet all produce heat, a smell and eventually a trip. It ranks fourth rather than higher: real on this platform, and less common than the causes above it.
Check in this order, and why
- Look at the panel on the cavity wall for scorching or a hole — Visible with the door open and a torch, and it is a cheap part. It leads because it tests the top-ranked cause.
- Take everything out and look for metal trim, foil or a tie — Ten seconds and it resolves most arcing complaints. Tests the second-ranked cause.
- Unplug it and see whether the circuit then holds with everything else running — One test separates the appliance from the installation. Tests the third-ranked cause.
What an owner can safely check
- Clean the cover gently if it is only soiled; if it is scorched through, stop using the appliance — A holed cover is a replacement, and it is not a wipe-and-carry-on item.
Tools: cloth, torchAbout 15 min
- Remove the item, then inspect the cavity walls and the turntable for burn marks — A burn mark left behind becomes the starting point for the next arc.
Tools: torchAbout 10 min
- STOP before anything behind a fixed panel, and note what each check showed — Everything past this line is measured rather than looked at, and measuring it safely needs the power off and a meter.
Stop first, diagnose second
Safety
Unplug the appliance. A microwave holds a high-voltage capacitor charge after it is unplugged, so nothing here involves removing the cabinet — that is a trained job with a discharge tool, and there is no safe owner version of it.
When to call a technician
Call, or replace the appliance, the moment the fault is anything other than a soiled waveguide cover or metal left in the cavity. An appliance that trips a breaker, arcs repeatedly after cleaning, or smells hot is not a watch-and-see appliance.
Nearest pages of this class
What this page does not cover
This page is written for the Miele microwave as a class, not for one family or one model line. Where a family- or model-specific page exists for your unit, that page is the more specific answer and the one to read first. Deliberately not here: part numbers, wiring colours and disassembly order, which differ by family.
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
