Marsh M. | Licensed Appliance Technician | 16 years experience | Perth & Melbourne
Quick answer: if your microwave runs but doesn’t heat, first check it isn’t on a low power level or in demo mode, and that the door closes fully. If a cup of water stays cold after a minute on full power, a high-voltage part such as the magnetron, diode, capacitor, door switch or inverter board has usually failed, and it needs a qualified technician.
The light is on, the turntable spins, the timer counts down, and your food comes out cold. It’s one of the most common microwave faults we see in our workshop.
Sometimes it’s a setting, and you can fix it in a minute. Sometimes a high-voltage part has failed, and that needs a technician. Start with the safe checks below. If they don’t solve it, we explain what has usually failed and whether it’s worth repairing.
5 safe checks before you call anyone
None of these involves opening the microwave.
1. Check the power level. If the microwave was last used to defrost or on a low setting, it may still be on low power. Food will only warm slightly. Select full power and try again.
2. Check for demo mode. Many microwaves have a demonstration mode for showrooms. The light, fan and turntable all work, but nothing heats. The display often shows “DEMO” or a similar code, or it may have been switched on by accident. A quick way to spot it: in demo mode the timer often counts down faster than normal. Your manual shows how to turn it off.
3. Do the water test. Put a microwave-safe cup of water in the centre and heat it on full power for one minute. Working normally, the water should be clearly hot. If it’s barely warm, the microwave isn’t producing heat, and the fault is internal. Never run the microwave empty to test it.
4. Check the door. Make sure the door closes fully and clicks shut. A door that doesn’t latch properly can stop the heating circuit from switching on, even when the light comes on.
5. Reset it. Switch off at the wall, unplug it for a few minutes, then plug it back in. This can clear an electronic glitch in the control board. Try a different power point too, rather than a power board.
If the water test still fails after all five checks, stop there. The remaining causes are all inside the microwave.
What usually stops a microwave from heating
A microwave heats food using a part called the magnetron, powered by a high-voltage circuit. If any link in that circuit fails, everything else keeps working, but no heat is produced.
- Magnetron: creates the microwave energy. When it fails, the microwave runs normally but doesn’t heat. It’s one of the most common causes. We recently replaced a failed magnetron in an LG built-in microwave in Balwyn North that had started making a cracking noise and giving off an electrical smell.
- High-voltage diode: works with the capacitor to power the magnetron. A failed diode often causes a loud hum and no heat.
- High-voltage capacitor: stores the charge that powers the magnetron. If it fails, the magnetron can’t run.
- High-voltage transformer: steps household power up for the magnetron. Less common, but a failure stops heating completely.
- Door switches: several switches confirm the door is shut before heating starts. A faulty one can stop heating, or blow the internal fuse so the microwave goes dead.
- Thermal cutout: a safety switch that cuts power if the magnetron overheats. A faulty one can stop heating or make the microwave cut out mid-cycle.
- Control board: a failed relay or circuit on the board can stop the heating circuit switching on, even though the display and timer work.
- Inverter board: many Panasonic and Sharp microwaves use an inverter instead of a transformer. When the inverter board fails, the result looks the same: no heat. On Panasonic models this often shows as an H97 or H98 code. Panasonic Australia’s support page says the code can follow a power surge, so switch off at the wall and back on first. If it returns, the power board or magnetron has usually failed. That was the case with a Panasonic combination microwave from Middle Park that we repaired in our workshop.
Several of these faults look identical from the outside, and sometimes two parts fail together. That’s why proper diagnosis matters before any parts are replaced.
From our workshop: a Sharp microwave came in running but not heating. Our technician replaced the control board, the usual cause on that model, but it still didn’t heat. Further bench testing found the inverter board had failed too. Two faults, one symptom. It’s why we test before and after replacing parts, rather than swapping one part and hoping.
Why you shouldn’t open a microwave yourself
Microwaves are among the most dangerous household appliances to repair. The high-voltage capacitor can hold a lethal charge even after the microwave has been unplugged. The circuit around the magnetron runs at thousands of volts.
Stop using the microwave and switch it off at the wall if you notice sparking, a burning smell, smoke, or a loud buzz with no heat. Don’t remove the outer casing. Diagnosing these parts safely needs a trained technician with the right equipment.
Is it worth repairing your microwave?
The honest answer depends mostly on what kind of microwave you have.
- A basic countertop microwave: if it cost less than the likely repair, replacing it often makes more sense. We’ll tell you that rather than sell you a repair.
- A mid-range or inverter countertop model: often worth assessing, especially if it’s only a few years old. Many faults come down to one or two parts.
- A built-in or combination microwave oven (for example Miele, Bosch, Neff or Smeg): usually worth repairing. These cost far more to replace and are fitted into your cabinetry, so a replacement may not fit the same space.
Age, condition and parts availability also matter. We assess the microwave first and give you a clear quote, so you can decide with the numbers in front of you.
How we help
- Workshop testing: built-in and many benchtop microwaves are repaired at your home. When a fault needs bench testing, we take the microwave to our workshop, and for built-in models we can fit a loan microwave in the meantime.
- Clear quote first: nothing is replaced until you’ve approved the price.
- Warranty: repairs come with 3 months labour and 12 months parts warranty.
- Straight advice: if it’s not worth repairing, we’ll say so.
For workshop repairs, see our small appliance repairs in Melbourne or call 1300 834 633.
Frequently asked questions
Why does my microwave run but not heat?
First check that it isn’t on a low power level or in demo mode, and that the door closes fully. If a cup of water still stays cold after a minute on full power, a part in the high-voltage circuit has usually failed. The most common are the magnetron, diode, capacitor or control board. Our technicians test each part in the workshop, so only the faulty ones are replaced.
Can a microwave stop heating because of the door?
Yes. Microwaves use several door switches to make sure the door is closed before heating starts. If the latch is worn or a switch fails, the microwave may light up and spin but not heat. Some door switch faults also blow the internal fuse, so the microwave stops working completely. On a recent Miele benchtop microwave in Burwood, the door hook that engages the latch had broken. A new hook was fitted and the door closed and worked correctly again.
Is it safe to keep using a microwave that isn’t heating properly?
No. Switch it off at the wall and unplug it, especially if you notice sparking, a burning smell or a loud buzz. A high-voltage fault can get worse with use, and some parts hold a dangerous charge even when unplugged.
What should I do if my microwave is sparking?
Stop the microwave straight away and switch it off at the wall. Sparks can come from metal left inside, such as foil, a twist tie or a plate with a metallic rim. If there’s no metal inside, the cause is often a burnt or greasy waveguide cover (the small panel on the inside wall), or damage to the cavity paint. On a recent De Dietrich benchtop microwave in Hawthorn East, two waveguide covers had shorted out and were causing sparks every time it ran. Replacing both covers fixed it. LG Australia’s guide to microwaves sparking during heating covers the common causes. Repeated sparking, smoke or a burning smell can also mean a high-voltage fault, so don’t keep using it.
How long should a microwave last?
Many microwaves last around 8 to 10 years, depending on how often they’re used and how well they’re kept clean. Built-in and premium models are often worth repairing beyond that, because replacement is expensive.
Is it cheaper to repair or replace a microwave?
For a low-cost countertop microwave, replacement is often cheaper. For built-in, combination or premium models, repair usually costs far less than replacing and refitting. We quote before any work starts, and repairs are backed by 3 months labour and 12 months parts warranty.
