What does it mean when a microwave runs but does not heat?
The light, turntable and fan run on ordinary mains power. The heat comes from a separate high-voltage circuit that powers the magnetron. When the microwave runs but stays cold, the low-voltage side is working and the high-voltage side is not. That narrows the fault down to a handful of parts.
Common causes
These are the usual causes, starting with the ones you can check yourself.
At a glance
| Cause | Who fixes it |
|---|---|
| The power level or programme is set wrong | You can check this |
| A door switch is worn | Technician |
| The high-voltage diode has failed | Technician |
| The magnetron has failed | Technician |
| The capacitor or transformer has failed | Technician |
- 1
The power level or programme is set wrong
You can check thisA defrost or low power setting heats very slowly, and some models have a demo mode that runs the turntable without heating. Check the power level and the manual for a demo mode before anything else.
- 2
A door switch is worn
TechnicianTwo or three switches tell the microwave the door is shut. If one is worn, the microwave may run the light and turntable but block the magnetron. A door that does not close firmly or needs a push to start is a sign. Testing and replacing the switches is a technician job.
- 3
The high-voltage diode has failed
TechnicianThe diode works with the capacitor to double the voltage for the magnetron. A failed diode is one of the most common reasons a microwave stops heating, and it often goes with a loud hum.
- 4
The magnetron has failed
TechnicianThe magnetron makes the microwaves that heat the food. Age, running the microwave empty and blocked air vents all shorten its life. A failed magnetron needs replacing, and on older or cheaper models that may cost more than the microwave is worth.
- 5
The capacitor or transformer has failed
TechnicianThe high-voltage capacitor and transformer feed the magnetron. Either can fail on its own. These are the most dangerous parts in the microwave and must only be tested by a technician with the right equipment.
What you can safely check yourself
- Check the power settingSet full power and heat a cup of water for one minute. It should be clearly hot.
- Check the doorOpen and close the door firmly. Look for a cracked latch, a bent hinge or food stuck on the door seal.
- Check the plug and socketPlug the microwave into a different wall socket, not an extension lead or multiplug.
- Keep the vents clearMake sure nothing blocks the vents on the sides, top or back. An overheating microwave can cut out.
When to stop and call a technician
You hear a loud hum or buzz, see sparks or smell burning, the microwave trips the power, or the fault is still there after these checks. Do not remove the outer casing. The high-voltage capacitor inside can hold a lethal charge after the microwave is unplugged.
Still having trouble with your microwave?
Our technician comes to your home, usually the same day. The R450 call-out falls away if you go ahead, and you get a fixed price in writing before any work starts.
How a technician diagnoses it
- Safe dischargeThe technician unplugs the microwave and safely discharges the high-voltage capacitor before touching anything inside.
- Door switch testEach door switch is tested in the open and closed position.
- High-voltage parts testThe diode, capacitor, transformer and magnetron are tested one by one to find the failed part.
- Fixed priceYou get a fixed price in writing before any part is replaced.
Is it worth repairing?
Usually worth repairing when
- The microwave is a built-in or combination model
- The fault is a door switch or diode
- The casing, door and seal are in good condition
Consider replacing when
- The magnetron has failed on a basic countertop model
- The inside is rusted or the door is damaged
- Several high-voltage parts have failed together