Why is my microwave not heating the food?

Two quite different complaints share this description. One is a machine that runs, lights and turns but produces nothing, which is a fault. The other is food that comes out hot at the edges and cold in the middle, which is how microwaves work and is dealt with by technique. One test separates them in sixty seconds.

Safety

What you'll need

• A microwave-safe mug
• Cold water
• A kitchen thermometer, to make the test unambiguous (optional)

Step by step

1. Do the mug of water test.Half a mug of cold water in the centre of the turntable, one minute at full power. Water absorbs microwaves efficiently, so a working machine will make it distinctly hot. Unchanged water means no microwave output.
2. If the water heats, the appliance is fine.Your problem is uneven heating, which is inherent: the field forms hot and cold bands about six centimetres apart and the turntable averages them rather than removing them. The remedies are arrangement, lower power for longer, stirring, and standing time.
3. If the water does not heat, check the obvious first.Is the display counting down and the turntable turning? Is the machine actually starting rather than sitting in a paused state? Is a demo or shop mode set, which some models have and which runs the display and light with no output?
4. Check the door closes properly and latches fully.Interlock switches in the door frame tell the microwave it is safe to produce output. A door that is not closing fully, a warped frame, or a broken latch tab gives exactly this symptom. Look at how the door meets the frame all round.
5. Note whether it is silent or noisy while running.A working magnetron makes a distinct hum under the fan noise. A machine that runs quietly with only fan noise, or that hums and then trips its own thermal cut-out, gives an engineer a useful starting point.
6. Check the turntable is turning.A stalled turntable does not stop heating, but it turns even heating into scorched-and-frozen patches, which people describe as not heating. Debris under the roller ring or a dish fouling the cavity wall are the usual causes and both are fixable in a minute.
7. Stop at the casing.The likely faults — a door interlock switch, the high-voltage diode, the capacitor, the magnetron — all sit behind the casing with the capacitor, and that is a lethal component. There is no exception to this and no test worth the risk.
8. Decide repair or replace on the numbers.A magnetron plus labour on a freestanding microwave is very often close to the price of a new one, and the rest of the machine is the same age. On a built-in or a combination model the arithmetic can favour repair, because the appliance is much more expensive to replace.

Tips

• If food heats unevenly, arrange it in a ring with the middle empty. The centre of the turntable moves least relative to the field pattern and is the least evenly heated place in the cavity.
• Cover the food and add a splash of liquid when reheating. Most microwave disappointment is moisture loss rather than uneven heating.
• Take a photo of the rating plate before ringing for a repair. Model and serial number is most of the conversation, and it is on the back or inside the door frame.
• If the machine trips the household circuit as well as failing to heat, treat that as a separate electrical fault and do not keep resetting.

If it doesn't work