The short answer
Run a cup of room-temperature water for a short cycle. If it comes out unchanged, the high-voltage side is not producing and the unit needs a technician. If it warms slowly rather than not at all, look first at the supply and at the clearance around the unit.
What you are seeing
- Turntable turning, light on, food cold — The standard picture of a high-voltage fault.
- Food warm but far slower than it used to be — Look at the supply and the ventilation before the appliance.
- A hum that is louder than usual with no heating — The transformer is being energised without result.
- Heating stops after a few minutes every time — A thermal protector, often from a unit pushed back against a wall.
- A smell of hot electrical insulation — Stop using it now and read the electrical-safety page.
- Not this fault: nothing at all happens when start is pressed — Go to the will-not-start page.
Ranked for a counter unit
- Magnetron at end of life — First here. It fails gradually in some units and outright in others, and both end with cold food.
- High-voltage diode or capacitor failure — Second, and indistinguishable from the above without instruments.
- Thermal protector opening from poor clearance — Third: a unit backed against a wall recirculates its own hot air.
- Supply voltage sagging on a shared circuit — Fourth, and the leading cause of slow rather than absent heating.
- Transformer failure — Fifth, and usually announced by smell.
- Control relay not closing — Last, and the rarest.
Test in this order
- Do the water test properly, with a measured cup at room temperature — It separates no heating from slow heating, which are different pages of causes.
- Move the unit clear of walls and other appliances — Clearance is free and it addresses the thermal branch.
- Put the unit on its own wall socket without an extension lead — This is the whole supply branch in one step.
- Repeat the water test after both changes — A unit that now heats has told you the appliance was never the fault.
Owner-side steps
- Heat a measured cup of room-temperature water for a short cycle — Judge the result by touch, not by how hot a plate of leftovers feels.
Tools: measuring cupAbout 5 min
- Pull the unit forward so air can leave the rear and side vents — Counter units are routinely installed with no clearance at all.
About 10 min
- Plug it directly into a wall socket of its own — A shared lead is the usual cause of weak, slow heating.
About 10 min
- Clean the cavity and the waveguide cover face — Burnt residue on the cover degrades performance and can scorch.
Tools: cloth, dish soapAbout 15 min
- Repeat the water test and note the difference — Two comparable tests make a far better handover than one impression.
Tools: measuring cupAbout 5 min
- Stop at the case. It is not opened for this or any other fault — The stored charge inside is lethal and does not clear on unplugging.
Non-negotiable on any microwave
Safety
Unplug the appliance or switch off its breaker before removing any panel. A capacitor can hold a charge after the power is off, and in a microwave that charge can kill. Do not run the unit empty, do not use it with a scorched waveguide cover, and stop immediately if you see arcing or smell burning insulation.
When to hand it over
Call once a clean water test shows no heating with good clearance and a dedicated socket. Call at once if there is any burning smell, any arcing, or a breaker that trips when a cook cycle begins. Ask for the cause to be named before agreeing to a repair: on a counter-standing unit the common high-voltage repairs are frequently a large fraction of a replacement.
Other pages for this unit
What is out of scope here
Generalized across families, written for the counter-standing case; the companion page takes the mounted case, where ventilation dominates. Where a family-specific or model-specific page exists it is the more precise answer. Not covered: component ratings, discharge procedures, high-voltage measurements and anything requiring the outer case to be removed.
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
