Stop, then read this
Any burning smell, spark or breaker trip from a CHM microwave means the oven comes out of service before anything is diagnosed. On this appliance class the causes divide cleanly: arcing inside the cavity, which is often something as simple as a soiled waveguide cover, and a failure in the high-voltage section, which is not owner territory under any circumstances. The CHM designation spans fifteen model numbers in the table below. Which one you have decides the part number, not the repair — the fault below is the same across the line.
What you actually observe
- Scorch marks on the waveguide cover — The mica sheet on the cavity wall. Splatter soaks into it, chars, and then arcs where it has charred.
- Breaker or a protected outlet trips after a few minutes — Something is heating until it fails rather than shorting outright — a different investigation from an instant trip.
- Smoke from the vents with nothing burning inside — The fault is behind the cavity. Unplug it at the wall and do not restart it to see if it does it again.
- Plug or cord warm to the touch — A supply-side problem — a tired outlet, an extension lead, or a loaded circuit. Worth checking before the oven is blamed.
- NEAR MISS — a single spark from a foil-wrapped item — That is the food, not the oven. Remove the metal and inspect the cavity for damage before using it again.
Causes, most likely first on the CHM line
- Contaminated cavity surfaces — Splatter on the cover, the walls or the roof of the cavity carbonises, and carbon is where arcing starts. This is first by a distance on ovens that have not been cleaned recently.
- A short that only appears under load — A breaker that trips exactly when start is pressed points into the high-voltage section rather than at the control side, which is why nothing on the keypad will diagnose it.
- Ventilation and the thermal cutout — An oven that runs hot, smells, and then goes dead for a while has almost certainly opened a thermal protector. Clearing the grille addresses the cause; the cutout is the symptom.
- Circuit loading — A standard US household branch circuit is 15 A or 20 A at 120 V; a large appliance circuit is typically 30 A or higher at 240 V. A microwave on a shared circuit with a kettle or a toaster will trip one without any fault at all.
Check in this order, and why that order
- Unplug it before anything else — Unplug the appliance or switch off its breaker before removing any panel. A capacitor can hold a charge after the power is off. With a hazard symptom the plug comes out first and stays out.
- Open the door and look at the cavity — The waveguide cover, the roof and the walls. Charring, a burnt patch on the cover or bare metal where the coating has chipped explain most sparking with no tools at all.
- Check what else is on the circuit — Move the oven to a different circuit, on its own, before concluding the oven is at fault. Shared kitchen circuits trip honestly.
- Stop at the cabinet — A trip under load, smoke from the vents or a smell from behind the cavity is high-voltage territory, and that territory is closed to owners on any microwave.
Before you open anything
Safety
Unplug the appliance or switch off its breaker before removing any panel. A capacitor can hold a charge after the power is off. If there is smoke, a smell of burning plastic, or a breaker that will not hold, leave the oven unplugged and do not use it again before it is inspected. Never run a microwave empty, never bypass a door interlock, and never put a microwave on an extension lead — it is one of the heaviest single loads in a kitchen.
Models this page covers
Reference| Model number | Family key |
|---|---|
| CHM009 | commercial-chef:microwave:chm |
| CHM11GBS | commercial-chef:microwave:chm |
| CHM14110B6C | commercial-chef:microwave:chm |
| CHM14110S6C | commercial-chef:microwave:chm |
| CHM14110W6C | commercial-chef:microwave:chm |
| CHM16100B6C | commercial-chef:microwave:chm |
| CHM16100S6C | commercial-chef:microwave:chm |
| CHM16100W6C | commercial-chef:microwave:chm |
| CHM660B | commercial-chef:microwave:chm |
| CHM660W | commercial-chef:microwave:chm |
| CHM770B | commercial-chef:microwave:chm |
| CHM770SS | commercial-chef:microwave:chm |
| CHM770W | commercial-chef:microwave:chm |
| CHM990B | commercial-chef:microwave:chm |
| CHM990W | commercial-chef:microwave:chm |
Where the owner line is on these models
The line is the cabinet screws and it does not move. In front of them: cavity cleaning, the waveguide cover, the vents, the outlet and the circuit. Behind them: the transformer, capacitor, diode, magnetron and thermal cutouts, all of which are implicated by exactly the symptoms on this page. If the oven has tripped a breaker or smelled hot, leave it unplugged while you arrange the visit rather than testing it again. Whichever of the fifteen models in the table above you have, quote the full number from the rating plate when you call — the family letters alone will not get the right part.
Other CHM problems we cover
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
