Microwave Not heating but runningwhat to check
The light is on, the turntable is going round and the food comes out cold. This is the classic microwave failure and none of it is an owner repair.
Take care
Get a qualified appliance engineer to look at your microwave
We do not pass enquiries to a repair firm, so there is no form here. When you book a qualified appliance engineer, tell them the make, the model and the exact code or message on the display, and ask whether they carry parts for this make — that is what turns one visit into a repair rather than a quote.
What this means
Everything looks right and nothing gets warm. The manufacturers have a name for this presentation — Whirlpool’s service table lists it as "Oven seems working, food does not cook" — and it separates a microwave from every other appliance here, because the part that makes the heat is not a heating element you could reach and test. Microwave energy comes from a magnetron, and a magnetron needs several thousand volts to work. Getting there takes a small chain of parts: a transformer to step the mains up, a capacitor and a diode arranged to roughly double that voltage, and the magnetron itself.
Any one of those failing gives you exactly this symptom, because the rest of the appliance — the lamp, the fan, the turntable, the timer, the display — runs on ordinary mains voltage and carries on as though nothing has happened. Whirlpool’s own list of causes for it names a defective magnetron with a broken or shorted filament, a broken transformer earth wire, bad contact at the magnetron terminals, and a breakdown of the high-voltage diode, which they note gives a hum. That hum is worth knowing about: a microwave that has started making a loud buzz at the same time as it stopped heating is pointing at the diode or the capacitor rather than the magnetron.
What makes this fault different from a cold oven or a dishwasher that will not heat is that the diagnosis is genuinely not available to you. The parts are behind the cover, and the cover is the one place on this site where being careful is not enough, because the capacitor stores energy and keeps it after the appliance is unplugged.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Magnetron has failed | The commonest single cause, and the one manufacturers list first. Filament broken or shorted to the body of the tube. | Common |
| High-voltage diode has broken down | Named by Whirlpool with a distinctive symptom attached: it produces an audible hum while the food stays cold. | Common |
| Capacitor or transformer no longer delivering | The voltage-doubling pair sits between the mains and the tube, and a failure there starves the magnetron. | Occasional |
| Poor connection at the magnetron terminals | Listed as its own cause rather than as a variant of tube failure, and it is the cheapest of these to put right. | Occasional |
| Long-term damage from running it empty | The energy has nowhere to go with no load in the cavity, and repeated episodes shorten the life of the tube. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£110–£220
Time on site
10 minutes
A magnetron is typically £40 to £90 as a part, and with a visit the job usually lands between £110 and £220. A diode or a capacitor is a cheaper part on the same labour.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related microwave fault codes
- Ran the microwave emptyGet it checkedRunning a microwave with nothing in it is the one thing an owner can do that reliably damages it, and the service manuals put it in capitals.Related: magnetron and high voltage
- Microwave completely deadGet it checkedNo display, no light, no fan. On a microwave this is very often a safety system working as designed, and fitting a new fuse is the wrong repair.Related: same brand
- Is my microwave leakingGet it checkedThere is a published UK limit and a real test for this, and the door seal is what usually fails it. What there is not, is a way to check it at home.Related: same brand
- Sparking inside the microwaveGet it checkedBright arcing and a scorched smell. Look at the small panel on the cavity wall before you run it again — a burnt waveguide cover is the usual answer.Related: same brand
- Turntable not turningUsually simple to fixFood heats in patches and the plate sits still. Almost always the coupler, the roller ring or a plate seated wrongly, and mostly fixable in the cavity.Related: same brand
- E01, E02 or E03 on an own-label microwaveGet it checkedOne inherited fault table printed under five UK badges. If your microwave shows E01, E02 or E03, the brand on the front is probably not who wrote it.Related: same brand
Related guides
Common questions
Is a microwave that will not heat dangerous to use?
It is not producing microwaves, so the containment question does not arise. The practical answer is still to stop using it: whatever failed is on the high-voltage side, and running the appliance repeatedly to see whether today is different puts load on the parts around it.
How do I tell a magnetron from a diode?
By ear, roughly. A loud hum or buzz appearing alongside the loss of heating points at the diode or the capacitor — Whirlpool attach the hum to a diode breakdown specifically. A silent failure with everything else behaving normally is more typical of the tube. An engineer will confirm it properly, and neither is a guess worth acting on.
Is a magnetron worth replacing?
On a built-in or a good combination oven, frequently. On a countertop machine, the part plus a visit will usually cost more than a replacement microwave. That is a genuine judgement rather than a way of avoiding the answer, and it turns mostly on what your appliance cost.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.