A microwave that runs and lights up but doesn't heat food is almost always a failed magnetron, a blown high-voltage diode, or a failed door switch — all routine repairs if the unit is worth repairing.
A microwave that runs (turntable spins, light turns on, fan runs) but doesn't heat food has lost its high-voltage path. A short list of parts is responsible.
The magnetron is the part that actually generates the microwaves. A failed magnetron is the most common single cause of a no-heat complaint on microwaves five-plus years old. The only safe diagnosis is with a multimeter; the only safe repair is by a technician — a microwave's high-voltage capacitor holds a potentially lethal charge for minutes after unplugging and has to be discharged properly before any internal work.
The high-voltage diode and high-voltage capacitor work with the magnetron to create the power the magnetron needs. A failed diode produces exactly the same symptom as a failed magnetron from the outside, and a diode is a $30 part versus a $100-plus magnetron. We test the diode first.
The door switches — typically three switches that all have to close for the microwave to power the magnetron — are the third common failure. A worn or broken monitor switch often blows the line fuse when it fails; the unit goes completely dead rather than running without heating.
If the microwave is over ten years old and a magnetron replacement is quoted, replacement of a basic countertop unit usually wins. For built-ins, trim-kit units, and premium models where a replacement unit costs $800-plus, a $300 to $450 repair is the better call.