A microwave that runs but won't heat has a failed high-voltage part — usually the magnetron, the diode, or the capacitor. Important: these hold a lethal charge even unplugged, so this is mostly a pro-or-replace situation.
Unlike most appliances, a microwave is genuinely dangerous to open. The high-voltage capacitor can store a lethal shock for a long time after unplugging. You can safely diagnose whether it's heating, but the actual repair belongs to a technician — and on many microwaves, replacement is the smarter economic call.
Check these in order. The first accounts for most cases.
The magnetron generates the microwaves that heat food. When it fails, the unit runs normally but produces no heat, sometimes with a loud hum.
The high-voltage diode helps power the magnetron. A failed diode stops heating and may cause humming or a blown fuse.
The capacitor in the high-voltage circuit can fail and stop heat output. It also stores a dangerous charge — never touch it.
A faulty door interlock can stop the microwave from energizing. This is one of the few parts a technician checks first.
Follow these in order. Stop as soon as the problem clears.
Microwave one cup of room-temp water for one minute. If it's still cold, the unit truly isn't heating (not just uneven food).
The high-voltage capacitor can hold a lethal charge even after unplugging. Opening the cabinet is not a safe DIY step.
Confirm the door closes fully and latches, and that you're not using a low-power setting. These are the only owner-safe checks.
Magnetron/diode/capacitor repair by a technician often approaches the cost of a new microwave, especially for countertop models. Over-the-range units are more often worth repairing.
If you want it fixed, have a qualified tech discharge the capacitor and replace the failed HV component — this is genuinely unsafe to DIY.