A microwave that runs through its cycle — turntable spinning, light on, timer counting — but produces no heat has a failure in the high-voltage circuit that powers the magnetron. This is distinct from a microwave that won't power on at all.
The magnetron is the component that generates the microwave energy that heats food. It fails with age or after power surges. The high-voltage diode converts power for the magnetron and is a common failure point — a failed diode often produces a loud buzzing sound along with no heating. The high-voltage capacitor stores and releases charge for the magnetron; if it fails, the magnetron gets no power.
**Safety note:** Microwave capacitors hold a dangerous charge — up to 6,000 volts — even when the unit is unplugged. Microwave repair requires proper discharge procedures before any internal inspection. Do not attempt to open a microwave for self-diagnosis.