The short answer
A Whirlpool over-the-range microwave that runs but does not heat has usually lost its magnetron, but before assuming that, check the grease filters and the vent path — a thermal cut-out opening produces heating that starts and then quits, which is a different pattern worth spotting. The filters are the one owner fix here. Anything behind the cabinet is not owner work, because of the capacitor inside.
What you observe
- Light, fan and turntable work, food stays cold — The signature of a high-voltage-side failure.
- It heats for a while then stops heating — A thermal cut-out opening. Check the filters and vents.
- Heating faded over weeks before it stopped — A magnetron reaching the end of its life.
- A new loud hum, or the breaker trips — Diode or capacitor. 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 an over-the-range unit works it harder than any other installation. It fades before it dies, which is why the complaint often begins as weak heating.
- Thermal cut-out opened by greasy filters or blocked vents — Second, and higher here than on any other platform: the unit sits in the cooktop plume, so the filters load fast and the vent path narrows. This is the one cause an owner can correct.
- High-voltage diode or capacitor failed — Third. Abrupt rather than gradual, usually with a loud 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 the heating circuit 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.
- Note whether it never heats or heats then stops — The second pattern moves the thermal cause to the top.
- Wash the grease filters and clear the vent path — The only cause an owner can correct, and it is real here.
- Check the power level is not set low — Left until here but still worth a moment.
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
- Time how long it heats before stopping, if it does — A consistent time points hard at a thermal cut-out.
About 15 min
- Unplug, slide out the grease filters and degrease them — Hot soapy water. Dry fully before refitting.
Tools: degreaserAbout 25 min
- Clear the exhaust path above and behind the unit — A recirculating unit vents through the top grille; keep it clean.
Tools: clothAbout 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 filters or vents. A capacitor can hold a charge after the power is off, and in a microwave that stored charge is capable of killing — this is the one appliance where the internal repair is off limits regardless of skill. Never run the unit empty, and never run it once you have smelled burning or heard arcing. Do not attempt to take an over-the-range unit off its bracket alone; they are heavy and awkward.
When to call a technician
Call once the filters and vents are clean and a full-power cup of water still comes out unchanged, 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. Ask for the parts price when you book: on an over-the-range unit, fitting costs usually make repair the cheaper option, but it is worth confirming.
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 and airflow figures, bracket dimensions and capacitor ratings, all of which differ between the over-the-range, built-in and countertop families.
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
