Almost always a failed magnetron (the high-voltage vacuum tube that generates the microwave energy). Magnetron replacement runs $200 to $400 including labour. Diagnosis is straightforward with proper test equipment.
Secondary causes: a failed high-voltage capacitor ($150 to $250), a failed high-voltage diode ($120 to $200), or a failed high-voltage transformer ($200 to $400). Any of these prevents the magnetron from producing microwave energy.
Because microwave repair involves high-voltage components (the capacitor can hold a lethal charge even when unplugged), this is not a homeowner repair category. Professional service with proper discharge procedures is required.









