The short answer
What you are seeing on a Thor microwave most often comes back to blocked vents overheating the electronics, with a shared or overloaded circuit behind it. Work the order below rather than the price list: the cheap causes here are also the common ones.
What you are seeing
- The breaker trips when the oven starts heating — The load arrives only when the magnetron fires, so the fault shows at that moment.
- The plug or the cord is warm to the touch — A supply-side fault that will not improve on its own.
- NEAR MISS: the oven will not heat with no smell or trip — That is a heating fault and a different page.
What it usually turns out to be, in order
- Blocked vents overheating the electronics — The cooling air keeps the magnetron and the transformer in range. Blocked, everything runs hotter and insulation suffers. It leads on Thor: a built-in cavity vents through a path the cabinet defines, and that path blocks before anything else does.
- A shared or overloaded circuit — A microwave draws close to a whole circuit when it heats, so it trips as soon as it shares one with another load. Second here: nothing else can be judged until the machine is genuinely receiving power.
- Arcing from metal or a damaged waveguide cover — Foil, a metal-trimmed dish or a burnt waveguide cover concentrates the field and arcs, which damages the oven fast. Third, once the two above are clean: heat is made by one circuit and protected by another, and either can stop it.
- A failing capacitor, diode or transformer — A high-voltage component breaking down draws hard and trips the supply, and it is what remains once the circuit is clean. Last, and the order is the point: nothing else can be judged until the machine is genuinely receiving power.
The order to work in
- Check the vent grilles and the clearance around the cabinet — A hot cabinet and a burning smell travel together.
- Try the oven alone on its own circuit with nothing else drawing on it — The cheapest possible test and a very common answer.
- Look at the waveguide cover on the cavity wall for burn marks — Burnt or missing, it arcs every time it runs.
What an owner can safely check
- Clear the vent grilles and give the oven the clearance the maker asks for — A built-in unit in a sealed cavity is the usual version of this.
Tools: a vacuum, a soft brushAbout 20 min
- Plug the oven directly into an outlet with nothing else on the circuit and test again — A standard household branch circuit is 15 A or 20 A, and a heating microwave wants most of one.
About 15 min
- Stop the oven at the first spark, then check for metal in the cavity and look at the waveguide cover — Never run it with the cover missing or burnt through.
Tools: a torchAbout 15 min
- STOP HERE: unplug the oven and stop using it. The capacitor holds a lethal charge even unplugged — No owner check past this point, at any skill level.
Before you open anything
Safety
Unplug the microwave before removing any panel. The high-voltage capacitor inside holds a lethal charge after the power is off and it does not discharge on its own. Never defeat, tape or bend a door interlock to make the oven run with the door open, and never run it empty. If the door, the latch, the seal or the window is damaged, stop using it: that is the barrier, and there is no safe way to test it at home.
When this stops being a do-it-yourself job
Stop using the oven and call straight away for a burning smell, a warm plug, repeated arcing or a breaker that trips on a dedicated circuit. None of these improve with time. A standard household branch circuit is 15 A or 20 A, and an oven that trips one on its own is reporting an internal fault, not being fussy.
Closest pages to this one
What this page leaves to a more specific one
This page covers the Thor microwave as a class, not one family or model line. Where a family-specific page exists it is linked below. Deliberately not here: fuse ratings, component test procedures and anything involving the high-voltage side, which no home guide should carry.
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
