The symptom: it runs, but stays cold
A microwave that will not heat can still look alive. The turntable spins, the interior light comes on, the fan hums, and the timer counts down. The food stays cold. That split is the most useful clue.
It suggests that the controls and low-voltage side are working while the high-voltage circuit that generates microwave energy is not.
The three usual suspects
The magnetron. This part produces microwave energy. When it fails, the unit can operate normally with zero heat. A magnetron often lasts eight to ten years. A buzz or loud hum may appear before failure.
The high-voltage diode. The diode helps raise voltage for the magnetron. A failed diode can leave the unit running without heat, blow a fuse, or create a loud buzz or burning smell.
The high-voltage capacitor. The capacitor works with the diode and magnetron. It is less often the first failed part, but it is the reason the inside of a microwave is not a casual DIY workspace.
A fourth possibility is a door interlock switch. The unit refuses to heat if it cannot read the door as fully closed. A worn switch or cracked holder can create a no-heat symptom even when the high-voltage parts are intact.
What you can safely check first
- Put one cup of water inside and run the unit for one minute. Do not run it empty.
- Check the power-level setting. Level one or two can heat very slowly.
- Make sure the door closes flush and that nothing blocks the latch.
- Unplug the unit for sixty seconds, then test again.
If the water stays cold after those checks, the fault is internal.
Why this is a stop-and-call safety issue
The high-voltage capacitor can store a charge strong enough to injure or kill, even after the microwave is unplugged. It must be discharged with the right equipment before the cabinet is safe to touch.
Do not remove the outer case, test the magnetron, or work on the diode or transformer at home. For microwaves, replacement is usually the honest answer when an internal fault appears.
Repair or replace
Use the full replacement cost, not just the sticker price:
- Countertop under about $200 new: replacement usually wins because an internal repair adds parts and labor.
- Over-the-range, built-in, or drawer unit: a typical Bay Area replacement can be about $600 to $1,500 after the unit and installation are considered. Compare installation and cabinet work before buying parts.
- Microwave-convection combo or wall unit: matching the opening and installation can make replacement more involved, but internal high-voltage work is still not a safe DIY project.
If the microwave sparks, smokes, or smells like burning plastic, stop using it and unplug it at the wall.
Make the replacement decision
Microwave repair is not a service ADRIUM offers. For a safe explanation of symptoms, replacement planning, and other home-maintenance questions, see the Home Service Pros information resource.
FAQ
Why does my microwave run but not heat? The low-voltage side works while the high-voltage circuit has failed. The magnetron, diode, or capacitor is a likely suspect.
Should I repair it or buy new? For a cheap countertop unit, buy new. For a built-in or over-the-range model, compare the full replacement and installation cost before deciding.
Can I do it myself? Not safely. The capacitor can hold a lethal charge after unplugging.
What should I check first? Test a cup of water, confirm the power level, inspect the latch, and reset power for sixty seconds. Do not open the case.