This is the category where we turn down the most work, and it is worth explaining why before you spend anything.
A microwave that runs but does not heat has a fault in its high-voltage section: the magnetron, the high-voltage diode, the capacitor, the transformer, or a door interlock that has stopped proving closed. Some of those are twenty-dollar parts. Some are most of the value of the machine. And the cabinet the microwave sits in decides the answer more than the fault does.
First, rule out the cheap non-faults
Two things that get reported as failures and are not:
It heats, but slowly. Run a proper load test before concluding anything: one cup of cold tap water in a glass measuring jug, two minutes on high. It should come out at a rolling boil or very close. Water that ends up merely hot means genuine power loss. Water that boils means the machine is fine and the complaint is about how it heats — usually a turntable that has stopped rotating, so the food sits in a standing-wave null and heats unevenly.
The panel lights and the fan spins, but nothing happens on start. Check the door. Every microwave has three interlock switches in series that must all confirm the door is shut. One sticky switch, or a latch hook worn round by twenty years of a slightly-slammed door, and the machine will light up, ventilate, and refuse to cook. This is a genuinely common and genuinely cheap repair.
Why we will not open a countertop microwave
The high-voltage capacitor in a microwave holds a lethal charge after the machine is unplugged. It has a bleeder resistor to discharge it, and bleeder resistors fail silently. Anyone working in there discharges it manually first, every time, no exceptions.
That is a real risk, and on a countertop unit it is a risk taken for a machine that costs very little to replace outright. There is no honest version of a service call that ends with you paying more than the appliance is worth. We say so on the phone and we do not come out.
Over-the-range is where the math flips
A microwave-hood combination is a different object. It is far dearer to replace, it is screwed to studs and hung on a mounting plate, it is vented into the wall or the roof, and swapping it means two people, a cabinet-width match and often a new vent transition. Repairing a modest fault in a unit like that is straightforwardly the right call.
The common failures we actually find in these:
- •Door interlocks, worn by being closed with a hip while both hands are full. Cheap.
- •The high-voltage diode, which fails shorted and usually takes the fuse with it. Cheap, and the most common true "runs but cold" cause.
- •The magnetron, which is the expensive one and is worth doing on a good unit and not on a bad one.
- •The grease filters and the blower, which is not a heating fault at all — it is why the kitchen fills with smoke, and it is maintenance.
That last one matters more in Boston than people expect, because so many kitchens here are small, closed rooms in old buildings where the hood is the only ventilation in the room. A blocked microwave-hood filter is not cosmetic when the kitchen has one window painted shut since the Carter administration.
Built-in and trim-kit units
Renovated kitchens in Brookline, Newton, Cambridge and the Seaport increasingly use a microwave built into a tall cabinet with a trim kit, or a microwave drawer. These are the most expensive microwaves in any house by a wide margin. They are repaired, not replaced, essentially every time — and the cabinet opening is usually cut to that exact model, so a replacement means either the identical unit or a carpenter.
The honest summary
| What you have | What we usually say |
|---|---|
| Countertop, any fault | Replace it. We will not charge you to look. |
| Over-the-range, interlock or diode | Repair. Comfortably worth it. |
| Over-the-range, magnetron, unit under 8 years | Usually repair. |
| Over-the-range, magnetron, unit over 12 years | Usually replace. |
| Built-in or drawer, any fault | Repair. |
Sparking, burning smell, or a noise that changed
Stop using it and unplug it. Arcing inside the cavity is usually a burnt waveguide cover — a small mica panel that is a genuinely trivial part — but it can also be a failing magnetron, and continuing to run it turns a cheap repair into an expensive one. A microwave that has started making a loud hum it did not used to make is telling you the transformer is unhappy. Neither of those is a wait-and-see.
If it is over-the-range or built-in, that is worth a call. If it is a countertop, that is worth a trip to the shop, and we would rather tell you that than sell you a visit.