The short answer
When a Nexel microwave runs, lights and turns but leaves the food cold, the answer is usually a thermal cutout opening because the box is being worked beyond its ventilation rather than a dead magnetron. This is a commercial machine, built for a duty cycle a household unit never sees, and that is what sets the order below. These sit on a stainless bench in a working kitchen, surrounded by steam, grease and everything else that a service line throws at them.
What you observe
- The light, the fan and the turntable all work and the food stays cold — Everything except the heat is behaving, which is the whole complaint.
- It heats for a short while and then stops heating — A pattern rather than a failure, and it points somewhere quite specific.
- It works from cold and not once it has been busy — A machine that recovers after standing is describing a thermal cutout.
- Heating is weak rather than absent — Food is warm at the edges after a long run instead of hot through.
- NEAR MISS — there is a loud hum and the breaker trips — That is the high-voltage section, not a heating complaint, and it needs the electrical-safety page for this appliance.
Causes, most likely first on this platform
- A thermal cutout opening because the box is being worked beyond its ventilation — Ranked this high because it is the failure a commercial box is set up for: back-to-back cycles, a vent facing a wall or a filter clogged with kitchen grease, and the cavity thermostat opens to protect the magnetron. On a service line the box is surrounded by steam and airborne grease all day, which is what moves this up the list.
- A door interlock switch that has failed open — Very common on a box that is opened and shut hundreds of times a shift.
- The magnetron or its high-voltage supply — A tired magnetron heats weakly before it stops altogether, and a failed diode or capacitor in the high-voltage circuit stops it outright, sometimes with a loud hum or a tripped breaker.
- A low power level, a defrost programme or a timer-only mode — Worth clearing in seconds because it costs nothing: a power level left low from the last job, a defrost programme, or a mode that counts down without firing.
Check in this order, and why
- First: let it stand unplugged until it is cold, then try it — Ranked here because a box that works cold and fails warm has already told you it is a thermal cutout. In a working kitchen the grease gets everywhere first, so this is where to look first.
- Second: press the door firmly closed and start it again — If it fires when the door is held tight, the interlocks are the cause and nothing else needs investigating.
- Third: listen for a change in the hum when heating should start — Only meaningful once the door and the ventilation are cleared — and it ends the owner-side investigation.
What an owner can safely check
- Clear the vents and any grease filter, then let it stand cold — A machine that works cold and fails warm has already told you it is a thermal cutout.
Tools: degreaser, clothAbout 30 min
- Press the door firmly closed and start again — If it fires while held, the interlocks are the cause and nothing else matters.
About 5 min
- Read the power level and the programme before starting — A level left low, or a defrost programme, produces exactly this complaint.
About 2 min
- STOP HERE — do not open the cabinet — The capacitor in the high-voltage section holds a charge capable of killing after the oven is unplugged. There is no owner check behind that panel at any skill level.
Before you open anything
Safety
Never open the cabinet of a microwave oven. Unplugging it does not make it safe: the capacitor in the high-voltage section stores a charge capable of killing long after the power is off, and there is no owner check behind that panel at any level of skill. Never defeat or tape a door interlock, never run the oven empty, and stop using it immediately if the door, the inner panel or the perforated screen is damaged.
When to call a technician
Call once the door, the ventilation and the supply are all clean, because what remains is the high-voltage section and that is never an owner job. Call immediately, and stop using the oven, if there is a loud hum, a burning smell or a tripped breaker alongside the loss of heat.
Nearest pages of this class
What this page does not cover
Read this as the class-level answer. It stops short of anything that changes model to model — connector pinouts, electrical test values, the sequence for pulling a panel and all part numbers — because getting those wrong is expensive. A family or model page, where one exists, is linked among the siblings and takes precedence over this page.
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
