The short answer
A Miele microwave that runs without heating has lost something on its high-voltage side, most often the magnetron. It is not owner-repairable, because the capacitor holds a lethal charge even unplugged. Confirm the symptom, rule out a low power level, and check the cabinet ventilation — then call, because on a built-in unit the repair is usually cheaper than replacing the appliance and its cabinet fit.
What you observe
- Light, fan and turntable work, food stays cold — The signature of a high-voltage-side failure.
- Heating faded over weeks before it stopped — A magnetron nearing the end of its life.
- It heats briefly, then stops mid-programme — A thermal cut-out opening. Check the cabinet vent path.
- A new loud hum, or the breaker trips — Diode or capacitor failing. Stop using the unit.
- NEAR MISS — the display is dark and nothing responds — Supply or line fuse. A different page.
Causes, most likely first on this brand
- Magnetron at end of life — First because it is a consumable and it is what fails on an otherwise sound built-in unit. It fades before it dies, so the complaint usually starts as slow heating rather than none.
- Thermal cut-out opened by a restricted cabinet vent path — Second on a built-in specifically: the unit relies entirely on the cabinet cut-out and trim for airflow, and a packed cupboard or a mis-fitted trim starves it.
- High-voltage diode or capacitor failed — Third. More abrupt than a magnetron, usually with a hum or a tripped breaker, and identical from the outside.
- Interlock switch failed in the wrong position — Last. A monitor or secondary interlock can block heating while everything else runs, which is why the door circuit is checked before anything expensive is condemned.
Check in this order, and why
- Heat a cup of water on full power — Confirms the symptom and separates weak from none.
- Check the power level is not set low — Free, and it explains a real share of these calls.
- Clear the cabinet around and above the unit — The only cause an owner can correct on a built-in.
- Note whether it never heats or heats then stops — Left until last because it tells a technician which half to test.
What an owner can safely check
- Run a cup of cold water on full power and feel it — Warm means weak heating; unchanged means none at all.
Tools: microwave-safe cupAbout 5 min
- Reset the power level to full and repeat — A level left low by a previous user is a real cause.
About 5 min
- Empty the cupboard above and beside the unit — A built-in vents through its trim and needs that path clear.
About 15 min
- Wipe the vent grilles clear of grease and dust — A blocked grille produces the heat-then-stop pattern.
Tools: degreaserAbout 15 min
- STOP HERE — do not remove the microwave cabinet — The high-voltage capacitor holds a lethal charge after unplugging.
Before you touch anything
Safety
Unplug the appliance or switch off its breaker before cleaning vents. A capacitor can hold a charge after the power is off, and in a microwave that stored charge is capable of killing someone who touches the terminals — this is the one appliance where the internal repair is off limits to owners regardless of skill. Never run the unit empty, and never run it once you have smelled burning or heard arcing. Do not try to draw a trimmed-in unit out alone.
When to call a technician
Call as soon as a full-power cup of water comes out unchanged, the power level is confirmed and the vent path is clear, because everything left is high-voltage work. Call before anything else if the unit hums loudly, arcs, smells of hot electrics or trips the breaker. Mention that the unit is built in when you book, because the cabinet fit affects both the access and the repair-versus-replace decision.
Nearest pages of this class
What this page does not cover
This page is written for the brand and the appliance class, not for one family or model line. Where a family- or model-specific page exists for your unit, that page is the more specific answer and you should start there. Deliberately absent: magnetron, diode and fuse part identifiers, power output figures, cabinet cut-out dimensions and capacitor ratings, all of which differ between the built-in, speed-oven and freestanding families.
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
