Microwave not heating

What it usually means

Start with the part that makes this appliance different from everything else in the kitchen. Inside a microwave is a transformer stepping mains up to a couple of thousand volts, and a capacitor that holds a dangerous charge after the plug is pulled. The service manuals all instruct engineers to discharge it before touching anything. So the rule on this page is not a formality: whatever the fault turns out to be, the outer casing stays on. Unplugging a microwave does not make the inside of it safe, which is not true of any other appliance in a domestic kitchen.

With that settled, the mechanism. Microwaves are produced by a magnetron, fed from that high-voltage circuit. Everything else — the light, the fan, the turntable, the clock, the beeps — runs from ordinary mains. When the magnetron or its supply fails, the appliance goes through a cycle perfectly and the food comes out cold, which is why "running but not heating" is the single most common microwave complaint and why it looks so much like nothing being wrong.

Before assuming that, rule out the free causes, because several of them produce identical symptoms. A power level left low is the first: at 20 or 30 per cent the magnetron is on for a few seconds in every thirty, and a mug of tea after two minutes feels untouched. A defrost programme does the same thing by design. Demonstration mode is a genuine setting on several makes, and manufacturers describe it plainly as the appliance running cold while appearing to work. On a combination oven, the wrong function is a classic: grill or convection selected means no microwave power at all, however normal the display looks.

Then there is the door, which is not a lid. It is part of the shielding that keeps the energy inside the cavity, and behind it sits a chain of interlock switches whose job is to prove it is shut before anything is energised. They are among the most frequently failing parts in the appliance, and depending on the design a failed switch produces either a machine that runs without heating or one that does nothing at all. This is also why a microwave with a damaged, distorted or dropped door should be taken out of use rather than persevered with.

One owner action genuinely destroys magnetrons: running the appliance empty. With nothing to absorb the energy it is reflected back into the tube. So if it stopped heating immediately after somebody ran it with nothing inside, or after a container boiled dry, the timeline is diagnostic. Arcing is the other thing worth knowing — the small board on the cavity wall is a waveguide cover, and when it chars it arcs, which is spectacular, common, and repairable for very little.

Finally, the arithmetic. A magnetron plus a call-out on a £90 countertop microwave is not worth spending. On a built-in unit, or an inverter model that cost several hundred pounds, it usually is. Deciding which of those you own before booking anybody is the most useful thing on this page.