The short answer
If the turntable turns, the light works and the food stays cold, the high-voltage side is not producing. That is a service diagnosis in every case. The one thing worth checking first is whether the unit heats briefly and then stops, which points at a thermal cut-out and at blocked ventilation above the range.
What you observe
- Everything runs normally and the food is cold — The classic presentation of a high-voltage fault.
- Food warms for a minute or two, then stops warming — A thermal protector opening, often from restricted airflow.
- A loud hum but still no heating — The transformer is energised and the magnetron is not producing.
- The unit trips the breaker when a cook cycle starts — Stop; this is the electrical-safety page, not this one.
- Heating is weak rather than absent — Same family, earlier; treat it as the same diagnosis.
- Not this fault: it will not start at all — That belongs on the will-not-start page.
Ranked for a mounted unit
- Thermal cut-out opening because of restricted airflow — First worth excluding on this install, because greasy filters and a blocked vent path are common and cheap to fix.
- Magnetron reached the end of its life — Second, and the most common outright failure in this family.
- High-voltage diode or capacitor failed — Third. The symptoms are identical to the above from the outside.
- High-voltage transformer failed — Fourth, usually with a smell that the owner notices.
- Door interlock chain satisfied for running but not for cooking — Fifth, and it presents as a unit that runs and never heats.
- Control board not commanding the cook relay — Last, and the rarest of the set.
The only order that helps here
- Test with a measured cup of water rather than a plate of food — It removes the food and the guesswork from the observation.
- Note whether it heats at all before stopping — Brief heating points at a thermal protector; no heating at all does not.
- Clean the grease filters and check the vent path above — It addresses the one cause an owner can address.
- Stop there — Everything else in this list is behind the case, which does not come off.
What is safe to do
- Heat a cup of water for a short cycle and feel the temperature change — The standard test, and it either warms or it does not.
Tools: measuring cupAbout 5 min
- Wash or replace the grease filters underneath the unit — A loaded filter starves the cavity of cooling air.
Tools: degreaserAbout 20 min
- Clear the vent path and check the damper above the unit moves freely — Restricted venting is the cause of most thermal cut-outs here.
About 20 min
- Let the unit cool fully and repeat the water test — A cold unit that heats again names the thermal branch.
About 45 min
- Stop at the outer case in every circumstance — The capacitor behind it can hold a lethal charge long after the plug is pulled.
Why this one is not a home repair
Safety
Unplug the appliance or switch off its breaker before removing any panel. A capacitor can hold a charge after the power is off; in a microwave that stored energy has killed people who opened the case with the unit unplugged. Never run the unit empty to test it, and never operate it if the door, its perimeter or the cavity shows scorching.
When to call
Call once the water test confirms no heating and the filters and vent path are clean. Everything remaining is high-voltage work with defined safety procedures. Call at once, and stop using the appliance, if there is a burning smell, visible arcing in the cavity, or a breaker that trips the moment a cook cycle starts. On many domestic units the cost of a magnetron or transformer replacement is worth weighing against a new appliance before committing.
Other faults here
Scope of this page
Generalized across families and written for mounted units; the companion page covers the same symptom on a counter-standing unit. A family-specific or model-specific page, where one exists, is the narrower answer. Not here: magnetron ratings, capacitor discharge procedures, filament voltages and any test that requires the case to be removed.
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
